Bacterial transformation
In fact, it results from DNA of a bacterial cell penetrating to the host cell and becoming incorporated
right into the genotype of the host.
The era stretching over 1940s witnessed and recognized that the prevailing inheritence in microorganisms
(bacteria) was adequately monitored and regulated basically by the same mechanisms as
could be seen in higher eukaryotic organisms.
Interestingly, it was duly realized that bacteria to designate a
‘useful tool’ to decepher the intricate
mechanism of
heredity as well as genetic transfer ; and, therefore, employed extensively in the
overall
genetic investigative studies.
Griffith’s Experimental Observations* :
Griffith (1928), a British Health Officer, carefully
injected mice with a mixture comprising of
two different kinds of cells, namely :
(
a) A few rough (i.e., noncapsulated and nonpathogenic) pneumococcal cells, and
(
b) A large number of heat-killed smooth (i.e., capsulated and pathogenic cells.)
Living smooth pneumococci cells
— usually causes pneumonia is human beings and a host of
animals.
‘Rough’ and ‘Smooth’
— invariably refer to the ensuing surface texture of the colonies of the
respective cells.
Consequently, the mice ultimately died of pneumonia, and
‘live smooth cells’ were meticulously
isolated from their blood. Thus, one may observe critically that there could be certain cardinal
factor exclusively responsible for the inherent
pathogenicity of the smooth bacteria ; and it had eventually
transformed these organisms into
pathogenic smooth ones.
Griffith also ascertained that the
transforming factor might have been sailed from the transformed
cells
right into their progeny (i.e., offspring), and hence the inheritence of the characteristicfeatures of a gene.
Transformation : Transformation
may be referred to as – ‘a type of mutation occurring in
bacteria and results from DNA of a bacterial cell penetrating the host cell, and ultimately becoming
incorporated duly right into the ‘genotype’ of the host’.
In other words,
transformation is the process whereby either ‘naked’ or cell-free DNA essentiallyhaving a rather limited extent of viable genetic information is progressively transformed from one
bacterial cell to another. In accomplishing this type of objective the required DNA is duly obtained from
the
‘donor cell’ by two different modes, such as : (a) natural cell lysis ; and (b) chemical extraction.
Methodology :
The various steps that are involved and adopted in a sequential manner are as
enumerated under :
(1)
DNA once being taken up by the recipient cell undergoes recombination.
(2)
Organisms (bacteria) duly inherited by specific characteristic features i.e., markers received
from the donor cells are invariably regarded to be
transformed.
Example :
Certain organisms on being grown in the persistent presence of dead cells,
culture filtrates,
or cell extracts of a strain essentially having a ‘close resemblance (or
similarity)’,
shall definitely acquire, and in turn would distinctly and predominantly transmit
a definite characteristic feature(s) of the
related strain (i.e., with close resemblance).
(3) DNA gets inducted
via the cell wall as well as the cell membrane of the specific recipient
cell.
(4)
Molecular size of DNA significantly affects the phenomenon of transformation. Therefore,
in order to have an extremely successful transformation of DNA the corresponding
molecularweights (DNA) must fall within a range of 300,000 to 8 million daltons.
(5) Importantly, the actual number of
‘transformed cells’ virtually enhanced linearly with definite
increasing concentrations of DNA. Nevertheless, each transformation invariably comes
into being due to the actual transfer of a single DNA molecule of the
double-strandedDNA.
(6) Once the DNA gains its entry into a cell,
one of the two strands gets degraded almost instantly
by means of the available enzymes
deoxyribonucleases ; whereas, the second strand
particularly subject to
base pairing with a homologous segment of the corresponding
recipient cell chromosome.
Consequently, the latter gets meticulously integrated into the
recipient DNA,
as illustrated beautifully in Figure 6.5.
(7)
Transformation of Closely Related Strains of Bacteria : In reality, the transformation of
closely related strains of bacterial could be accomplished by virtue of the fact that
complementary
base pairing
predominantly occurs particularly between one strand of the donor
DNA fragment
and a highly specific segment of the recipient chromosome.
However, the major steps involved in the
bacterial transformation have been clearly shown in
Examples :
The bacterial species which have been adequately transformed essentially include :
Bacterial species
: Streptococcus pneumoniae (Pneumococcus)
Genera
: Bacillus ; Haemophilus ; Neisseria ; and Rhizobium
Bacterial Transcription
Bacterial transcription
refers to the – ‘synthesis of a complementary strand of RNA
particularly from a DNA template’.
In fact, there exists
three different types of RNA in the bacterial cells, namely : (a) messenger
RNA ;
(b) ribosomal RNA, and (c) transfer RNA.
Messenger RNA (mRNA) :
It predominantly carries the ‘coded information’ for the production
of particular proteins from
DNA to ribosomes, where usually proteins get synthesized.
Ribosomal RNA :
It invariably forms an ‘integral segment’ of the ribosomes, that strategically
expatiates the cellular mechanism with regard to
protein synthesis.
Transfer RNA :
It is also intimately and specifically involved in the protein synthesis.
Process of Bacterial Transcription :
Importantly, during the
process of bacterial transcription, a strand of messenger RNA (mRNA)
gets duly synthesized by the critical usage of a
‘specific gene’ i.e., a vital segment of the cell’s DNA–as
a
template, as illustrated beautifully in Figure : 6.6. Thus, one may visualize the vital and important
‘genetic information’
adequately stored in the sequence of nitrogenous bases (viz., A, T, C and G) of
DNA, that may be rewritten so that the same valuable
‘genetic information’ appears predominantly in
the base sequence of mRNA.
Examples :
(1) In the
DNA replication phenomenon, it has been duly observed that a G in DNA template
usually dictates a C in the mRNA ; and a T in DNA template invariably dictates an A in the
mRNA.
(2) An A in DNA template normally dictates a uracil (U) in the mRNA by virtue of the fact that
RNA strategically contains U instead of T*.
(3) In an event when the template segment of DNA essentially possess the
base sequence ATGCAT,
consequently the
strategic newly synthesized mRNA strand shall predominantlywould bear the complementary base sequence UAC GUA.
Salient Requirements :
The salient requirements for the process of bacterial transcription
are as enumerated under :
(1) It essentially needs
two cardinal components, namely :
(
a) RNA– polymerase — an ‘enzyme’, and(b) RNA–nucleotides — a regular and constant supply.
(2)
Promoter — Usually transcription commences once the RNA polymerase gets strategically
bound to the DNA at a specific site termed as
‘promoter’.
(3) Precisely, only one of the
two DNA strands invariably caters as the particularly required
‘template for the synthesis
for a given ‘gene’.
Examples :
There are several typical examples that explains the above intricate phenomenon
vividly :
(
a) Just as DNA, the RNA gets synthesized duly and specifically in the 5′ ⎯→ 3′ direction.
Nevertheless, the
‘equivalence point’ (i.e., endpoint) for transcription of the gene is
signaled suitably by a
terminator segment present strategically in the DNA. Interestingly,
at this particular zone, one may observe the release from the DNA of these
two entities
prominently : (
i) RNA polymerase ; and (ii) newly generated single-stranded mRNA.
(
b) ‘Regions of genes’ present critically in ‘eukaryotic cells’ which essentially afford ‘coding’
for the respective
proteins are usually interrupted by the so-called ‘noncoding DNA’. Therefore,
the
‘eukaryotic genes’ are made up of ‘exons’ i.e., the specific segments of DNA
expressed appropriately ;
besides, ‘introns’ i.e., the designated intervening segments of
DNA
which fail to code for the corresponding protein. Besides, in the eukaryotic cell the
nucleus
predominantly synthesizes RNA polymerase from the entire gene–a fairly long
and continuous RNA product*
usually termed as the RNA transcript.
Mechanism :
The ‘elongated RNA’ is subsequently processed by a host of other enzymes
that particularly help in the removal of the
intron-derived RNA and also splice together the
exon derived RNA
thereby producing an mRNA which is exclusively capable of ‘directly
the on-going protein synthesis’.
Consequently, the RNA gracefully walks out of the nucleus,
and ultimately turns into a
mRNA of the ensuing cytoplasm.
Ribozymes
— are non protein enzymes (i.e., a RNA enzyme) duly obtained as a result of
certain enzymes which are actually cut by the RNA itself.
(
c) Importantly, in eukaryotic organisms, the ensuing transcription usually occurs in the
nucleus.
It has been observed that mRNA should be completely synthesized and duly moved
across the
nuclear membrane right into the cytoplasm before the actual commencement of
the phenomenon of
translation. Besides, mRNA is duly subjected to further processing
mode before it virtually gets out of the nucleus.
Summararily, the valuable genetic information, derived from
prokaryotes and eukaryotes,
pertaining to protein synthesis is stored meticulously in DNA, and subsequently passed
on to
mRNA during the phenomenon of ‘transcription’. Ultimately, mRNA prominently
serves as the
source of information for the required protein synthesis.
Bacterial Translation
Bacterial translation
may be defined as — ‘the specific process via which the critical
nitrogenous-base sequence of mRNA affords determination of the amino acid sequence of protein’.
One may precisely observe that in an organism that is particularly devoid of a
membraneenclosed
nucleus,
both ‘transcription’ and ‘translation’ invariably occur in the cytoplasm. Thus, in
an
eukaryotic organisms, the process of ‘translation’ actually comes into being in a situation whenmRNA gains its entry into the cytoplasm.
Stage-1 :
Various components that are essentially required to commence the ‘phenomenon of
translation’
first come together.
Stage-2 :
On the assembled ribosome a transfer RNA (tRNA) carrying the ‘first amino acid’
is duly paired with the start codon on the
mRNA ; and a ‘second amino acid’ being
carried by
tRNA approaches steadily.
State-3 :
Critical place on the chromsome at which the very first tRNA sites is known as the P
site.
Thus, in the corresponding A site next to it, the second codon of the mRNA pairs
with a
tRNA carrying the second amino acid.
Stage-4 : First amino acid
gets hooked on to the second amino acid by a peptide linkage
, and the first
tRNA gets released.
Note : Nucleotide bases are duly labeled only for the first two codons.
Stage-5 : Ribosome
gradually moves along the mRNA until the second tRNA is in the P site,
and thus the process continues.
Stage-6 : Ribosome
very much continues to move along the mRNA, and thus, newer amino
acids are progressively added on to the
‘polypeptide chain’ strategically.
Stage-7 : Ribosome
when ultimately gets upto the ‘stop codon’, the duly formed polypeptide is
released.
Stage-8 :
Last tRNA gets released finally, and thus the ribosome falls apart. Finally, the released
polypeptide
gives rise to an altogether new protein.
Process of Bacterial Translation :
The various steps encountered in the elaborated process of
‘bacterial translation’
are :
(1) Proteins are usually synthesized strategically in the
5′ → 3′ direction, as present in DNA
and RNA (
i.e., nucleic acids).
(2) First and foremost, the
5′ end of the specific mRNA molecule becomes associated with a
ribosome,
which being the major cellular machinery that predominantly helps to catalyze
the
‘protein synthesis’.
(3)
Ribosomal RNA [rRNA] : Ribosomes usually comprise of two subunits ; of which, one
being a special type of RNA termed as
ribosomal RNA (rRNA) and the other proteins. At
the very outset of the process of
bacterial translation, the two ribosomal subunits happen
to get closer
vis-a-vis the mRNA plus many other components engaged in this phenomenon.
(4) Even before the suitable amino acids may be joined together to yield a
‘protein’, they should
be adequately
‘activated’ by strategic attachment to transfer RNA (tRNA).
Example :
The following sequence :
AUGCCAGGCAAA
essentially contains
four codons (i.e., four sets of 3 nucleotides of mRNA) codefying for the
amino acids
viz., methionine (AUG), proline (CCA), glycine (GGC), and lysine (AAA).
•
In case, the bases are grouped in an altogether different manner, the ‘same sequence’ might
specify other amino acids.
Example :
AUGC CAG GCA AA
The above sequence duly encodes :
cysteine (UGC), glutamine (CAG), and alanine (GCA).
•
Likewise, AU GCC AGG CAAA
Would rightly encode
alanine (GCC), arginine (AGG), and glutamine (CAA).
Reading Frames :
In fact, all the above cited ‘groupings’ are known as reading frames.
Importantly, a particular
reading frame is invariably determined by the inherent strategic
position (status) of the
‘very first codon’ of the gene.
(5) The
transfer RNA [tRNA] molecules actually help to ‘read’ the so called coded message
located strategically on the
mRNA.
Anticodon : Anticodon
refers to ‘a set of three nucleotides, which is critically positioned on
one particular segment of each
tRNA molecule, that happens to be complementary to the
codon
specifically for the ‘amino acid’ being carried by the tRNA [see Fig. 6.8(c)].
(6) It has been duly observed that in the course of
‘translation’, the highly specific ‘anticodon’
of a
molecule of tRNA gets intimately H-bonded to the complementary codon strategically
located on
mRNA.
Example :
One may critically observe that a tRNA having the desired anticodon CGA
pairs
specifically with the mRNA codon GCU. Therefore, the eventual pairing of anticodon
and
codon may usually take place solely at two sites as indicated by the ribosome, such as :
(
a) The ‘A’ or ‘aminoacyl-site’, and(b) The ‘P’ or ‘peptidyl-site’.
(
a) The structure of tRNA is designated in 2D-form. Each ‘box’ represents a ‘nucleotide’.
The critical zones of H-bonding between ‘base pairs’ and ‘loops of unpaired bases’
i.e.,
a typical arrangement to be seen exclusively in RNA molecules.
(
b) Activation of ‘each amino acid’ by due attachment to tRNA.
(
c) ‘Anticodon’ by tRNA invariably pairs with its complementary codon strategically located
on an mRNA strand. The tRNA displayed specifically carries the amino acid
‘alanine’. The ‘anticodons’ are mostly represented and duly read in the 5
′ ⎯→ 3′ direction
; and, therefore, the anticodon for the amino acid ‘alanine’ may be read as
C–G–A.
[Adapted From : Tortora GJ
et al. : Microbiology : An Introduction,
The Benjamin and Cummings Publishing Co. Inc., New York, 5th edn., 1995]
Bacterial Conjugation
The copious volume of literature available in
bacterial morphology provides several elaborated,
authentic descriptions of
‘microscopic observations of cell pairs’ which were duly ascertained
and identified as indicators of
mating and sexuality in organisms. Lederberg and Tatum (1946) first
and foremost comfirmed the phenomenon of
conjugation* in E. coli by carefully mixing autotrophicmutants** and finally meticulously selected the rare recombinants***. In fact, they initially plated
aseptically the
E. coli mutants with triple and complementary nutritional requirements [i.e., abc
DEF × ABC def] upon minimal agar, and duly accomplished the desired
prototrophic bacteria*
[ABCDEF]. Nevertheless, these recombinants were found to be fairly stable ; and, therefore, adequately
propogated and raised at a frequency ranging between 10– 6 to 10– 7,
An additional supportive evidence to demonstrate that the specific development of the ensuing
‘protrophic colonies’
essentially needed the absolute cooperation of the intact organism of either
species (types), which was duly accomplished by the help of the
U-Tube Experiment. In actual practice,
neither the
culture filtrates nor the cell free culture extracts were found to be appreciable productive
in nature thereby suggesting that the
actual cell contact was indeed an absolute must.
Lederberg and Tatum further critically screened a good number of the
‘prototrophic colonies’ to
ascertain and confirm whether the said
‘conjugation phenomenon’ happened to be ‘reciprocal in
nature’.
However, their observations duly revealed that invariably most colonies did comprise of exclusively
one
particular class of recombinants thereby amply suggesting that the ensuing ‘recombination
in bacteria’
could be precisely of an ‘absolute unorthodox type’ in nature. Besides, an elaborative
further investigation showed that the prototrophs initially found to be of
heterozygous** in nature, but
later on got duly converted to the corresponding
‘haploids’***. It is, however, pertinent to state here
that these
‘investigative studies’ undoubtedly proved that bacteria predominantly possessed ‘sex’ that
eventually rendered them to the following
two vital and important characteristic profiles, namely :
(
a) amenable to the ‘formal genetic analysis’, and(b) revelation of the very existence of genetic material present in a ‘chromosomal organization’.
The
process of conjugation essentially suggests that :
(1) large fragments of DNA were adequately transferred from one bacterium to another in a
non-reciprocal manner, and
(2) such transfer invariably took place from a given point.
One may also critically take cognizance of the fact that the exact size (dimension) of DNA
transferred from one cell to another was found to be much larger in comparison to the corresponding
transformation. Certainly, the
process of conjugation proved to be much more an absolutely commendable
and useful technique for the so called
‘gene mapping’ in organism.
Donor Bacteria
i.e., such organisms that are responsible for transferring DNA, and
Recipient Bacteria
i.e., such organisms that are responsible for receiving DNA.
Bacterial Transduction
Bacterial transduction
may be defined as — ‘a phenomenon causing genetic recombination
in bacteria wherein DNA is carried from one specific bacterium to another by a bacteriophage’.
It has been duly observed that a major quantum of bacteriophages, particularly the
‘virulent’
ones, predominantly undergo a rather
quick lytic growth cycle in their respective host cells. During this
phenomenon they invariably inject their nucleic acid, normally DNA, right into the bacterium, where it
takes up the following
two cardinal steps :
(
a) undergoes ‘replication’ very fast, and
(
b) directs the critical synthesis of new phage proteins.
Another school of thought may put forward another definition of
bacterial transduction as —
‘the actual and legitimate transfer by a bacteriophage, serving as a vector, of a segment of DNA
from one bacterium (a donor) to another (a recipient)’.
Zinder and Ledenberg (1952) first and foremost discovered the wonderful phenomenon during
an intensive search for
‘sexual conjugations’ specifically amongst the Salmonella species.
Methodology :
The various steps that are involved in the bacterial transduction phenomenon
are as stated under :
(1) Auxotrophic mutants were carefully mixed together ; and subsequently, isolated the
prototrophic recombinant colonies from the ensuing
selective nutritional media.
(2)
U-Tube Experiment : The U-tube experiment was duly
performed with a
parental auxotrophic strain in each
arm (
viz., I and II), and adequately separated by a
microporous fritted glass (MFG) filter,
whereby the
resulting
‘prototrophs’ distinctly appeard in one arm
of the tube, as shown in Fig. 6.10.
As the MFG-filter particularly checked and prevented cellto-
cell contact, but at the same time duly permits the
‘free passage’
of fluid between the said
two cultures [i.e., strains I and II], it
may be safely inferred that there must be certain
‘phenomenon’other than the ‘conjugation’ was involved.
Besides, the process could not be radically prevented to
DNAase (an enzyme) activity, thereby
completely eliminating
‘transformation’ as the possible phenomenon involved for causing definite
alterations in the recipient
auxotrophs to prototrophs.
The bacteriophage was duly released in a substantial amount from a
lysogenic (i.e., recipient)
culture.
Thus, the emerging phage critically passed via the MFG-filter, and adequately infected the
other
strain (i.e., donor) lyzing it exclusively.
Finally, during the
‘replication’ observed in the donor strain, the ensuing phage adventitiously
comprised of the relevant portions of the
critical bacterial chromosome along with it. Eventually, it
gained entry
via the MFG-filter once again ; thereby taking with it a certain viable segment of the
respective donor’s
‘genetic information’ and ultimately imparting the same to the desired recipient
strain.
Nevertheless, the
‘bacterial transduction’ may be further classified into two sub-heads,
namely :
(
a) Generalized transduction, and
(
b) Specialized transduction.
Generalized Transduction
In a situation when practically most of the fragments pertaining to the
bacterial DNA* do get an
obvious chance to gain entry right into a
‘transducting phage’, the phenomenon is usually termed as
‘generalized transduction’.
Modus Operandi
: The very first step of the phage commences duly with the ‘lytic cycle’ whereby
the prevailing
‘viral enzymes’ preferentially hydrolyze the specific bacterial chromosome essentially
into several
small fragments of DNA. In fact, one may most conveniently incorporate any portion of
the
‘bacterial chromosome’ right into the ‘phage head’ in the course of the ensuing phage assembly ;
and, therefore, it is not normally associated with any sort of
‘viral DNA’.
Example : Transduction of Coliphage P1 :
In fact, the coliphage P1 can effectively transduce
a variety of genes
in the bacterial chromosome**. After infection a small quantum of the phages carry
exclusively the
bacterial DNA as shown in Fig. 6.11.
Figure : 6.11 clearly illustrates the following
salient features, namely :
•
Phage P1 chromosomes, after injection into the host cell, gives rise to distinct degradation
of the specific host chromosome right into small fragments.
•
During maturation of different particles, a small quantum of ‘phage heads’ may, in fact,
envelop certain fragments of the
bacterial DNA instead of the phage DNA.
•
Resulting bacterial DNA on being introduced into a new host cell may get integrated into
the
bacterial chromosome, thereby causing the transference of several genes from onehost cell to another.
It has been observed that the
‘frequency’ of such defective phage particles usually range
between 10
– 5 to 10– 7 with respect to corresponding ‘progeny phage’ generated. As this particular
DNA more or less matches the DNA of the newer bacterium thus infected, the
‘recipient bacterium’
shall not be rendered
lysogenic* for the respective P1 phage. Instead, the injected DNA shall be duly
integrated right into the chromosome of the available recipient cell. In this manner, the so called
‘genetic
markers’
duly present in the DNA would precisely detect the very presence of all defective P1 phages
essentially bearing the
E. coli DNA.
Advantages :
The various glaring advantages of the generalized transduction are as given
below :
(1) Just like
bacterial conjugation (see Section 2.10) and bacterial transformation (see Section
2.7) the
generalized transduction also caters for the typical ways for ‘mapping* bacterial
genes’,
by virtue of the fact that the fragments duly transferred by the bacteriophage
are invariably big enough to safely accomodate
hundreds of genes.
(2) To test actually the exact quantum of such
‘recombinants’ that have inherited from other
‘donor markers’
due to the growth occurring on other culture media.
(3) Strategic closeness of the
‘two markers’ on the bacterial chromosome ascertains the fact
that they would be inherited together more likely by the aid of a
single transducing phage.
Specialized Transduction
Based on enough scientific evidences it has been duly proved and established that the
‘bacterial
genes’
may also be adequately transduced by means of bacteriophage in another equally interesting andthought provoking phenomenon usually termed as ‘specialized transduction’. In fact, this phenomenon
confirms duly that certain
template phage strains may be capable of transferring merely a handful of
‘restricted genes’
belonging categorically to the ‘bacterial chromosomes’.
In other words, the ensuing phages particularly transduce exclusively such
bacterial genes that
are strategically positioned quite adjacent to the
prophage in the bacterial chromosome. Therefore,
this particular process is sometimes also referred to as
‘restricted transduction’. Interestingly, in an
event when such a phage duly infects a cell, it invariably carries along with it the specified
group of
bacterial genes
which ultimately turns out to be an integral part of it. Consequently, such genes may
recombine meticulously with the
homologous DNA of the prevailing infected cell.
Phage Lambda (
λ) of E. coli. : In a broader perspective, the most elaboratedly researched specialized
transducting phage
is duly represented by the phage lambda (λ) of E. coli. The exact location
of the ensuing
λ prophage present in the bacterial chromosome invariably lies between the bacterial
genes
gal and bio. It may be observed that whenever phages duly carrying either a gal or bio genes do
infect an altogether
‘new host’, then the desired recombination either with the gal or bio genes of the
respective may take place articulately. Fig. 6.12 depicts vividly the phenomenon of
specialized
transduction.
Salient Features :
The various salient features highlighting the process of specialized
transduction
in Figure 6.12 are stated as under :
(1) Practically ‘all phages’ which essentially carry certain
bacterial genes solely on account of
‘‘incorrect’’
excision are obviously found to the ‘defective’ with respect to certain highly
important functions.*
(2) Thorough passage
via the entire ‘replication cycle’ cannot be accomplsihed ; whereas, the
ensuing cell may suitably give rise to
certain phages, provided it is also duly infected witha rather ‘complete phage’**.
Bacterial Transfection
Bacterial transfection
refers to — ‘the infection of bacteria by purified phage DNA after
pretreatment with Ca
2+ ions or conversion to spheroplasts.*
Nevertheless, the wonderful discovery of
transformation critically revealed that ‘large molecular
weight DNA’
may also penetrate the cell walls of a plethora of so called competent bacteria.
In fact, Fraenkel-Courat
et al., (1957) amply demonstrated that the purified RNA meticulously derived
from the well known
tobacco mosaic virus was also found to be equally ‘infective’ in nature.
Since then, quite a few classical examples of the
‘infective characteristic features’ of the nucleic
acids
viz., DNA, RNA, have been adequately brought to light to the knowledge of the various
researchers. Foldes and Trautner (1964) proved to be the pioneers to exhibit and demonstrate explicitely
that the
‘protoplasts’ of organisms may also be duly infected even with the ‘purified nucleic acid’,
which phenomenon was baptized by them as
‘Transfection’. However, it was duly ascertained that the
‘competent cells’
exclusively were found to be sensitive and the infection was equally sensitive to the
enzyme
DNAase.
Consequently, further extensive and intensive studies do ascertain that
‘transfection’ is duly
extended to a plethora of other organisms as well.
Phage Conversion
Freeman (1951) critically took cognizance of the fact that in a specific condition
‘certain nontoxic
strains’
of the bacterial sp. Corynebacterium diphtheriae (causing the dreadful disease ‘diphtheria’
amongst children), are duly subjected to adequate treatment with a
‘phage suspension’ that has
been carefully obtained from a highly
‘virulent toxigenic strain’ of the same species, then a certain
proportion of survivors acquired the substantial capability of synthesizing the toxin and maintaining the
adequate desired
‘immunity’ particularly to the ‘lytic infection’ by the respective phage.
However, further follow up investigational studies have duly revealed that this specific sort of
typical conversion from a
nontoxigenic to a toxigenic strain was primarily caused on account of the
adequate establishment of the phenomenon of
‘lysogeny’**, and subsequently the inherent ability to
cause production of the
toxin was lost virtually along with the complete loss of the phage.
Conclusively, based upon the marked and pronounced presence of the correlation between
‘lysonization’
and ‘toxin generation’ the said phenomenon was approximately termed as — ‘lysogenic
conversion’.
Nevertheless, further elaborative studies distinctly helped to discover the fact that particular
virulent mutants of the converting phages
may also reasonably initiate the toxin synthesis, whichis known as ‘phage conversion’.
Example : Phage conversion
seems to be extraordinarily abundant and most frequent amongst
organisms.
The glaring production of the
somatic antigens in the Salmonella sp. by the help of various
recognized strains of the
‘Group E’ has been duly observed to be intimately related to the presence ofsome very specific bacteriophage genomes.