Biology Reference
In-Depth Information
Schneider and Haselkorn (1988) identifi ed restriction fragments of cyanophage N-1 DNA
which contains genes that transcribe early proteins in infected cells of
Anabaena
. The sequence of
two promoter regions to each other in the -35 and -10 regions were nearly found to be identical to
consensus sequence for promoters in
E
.
coli
. Muradov
et al
. (1990a,b) detected sequences homologous
to nitrogenase structural genes (
nifD
and
K
) of
Klebsiella
pneumoniae
in the DNA of various moderate
NP-1T cyanophage strains infecting
Nostoc
sp. N-39 lysogenized by these cyanophages. The
nif
genes
mentioned have not been detected in DNA of
Nostoc
sp. N-39 primary culture.
Szekers
et al
. (1983) showed that the endonuclease splits host DNA but not AS-1 phage DNA.
Further, AS-1 phage DNA was shown to be not only resistant to AS-1 endonuclease but also
to a number of restriction endonucleases, the recognition site of which contains a central G-dc
dinucleotide. The sites attacked preferentially by the AS-1 endonuclease are specially protected on
AS-1 DNA molecule. However, AS-1 DNA when inserted into plasmid pBR322 and cloned into
E
.
coli
was found to be susceptible to AS-1 endonuclease.
iii)
Disturbances in other metabolic features
:
Mann
et al
. (1975) for the fi rst time identifi ed that
accumulation of guanosine-3 diphosphate-5-diphosphate (ppGpp) occurred during energy limitation
in the cells of
A
.
nidulans
. Nitrogen starved (MSX-treated) and energy depleted cells of
A
.
nidulans
have been shown to accumulate large amounts of ppGpp. However, this accumulation of ppGpp
does not occur in the host cells infected with cyanophage AS-1 and also in
E
.
coli
cells infected with
T4 suggesting that it is the difference in control of nutritional or energy metabolism rather than
differences in ability to synthesize ppGpp (Borbely
et al
., 1980). Nitrogen starvation in
A
.
nidulans
led
to the accumulation of ppGpp (Friga
et al
., 1981). Now it is being increasingly realized that ppGpp
is centrally involved in the adaptation of cyanobacterial cells to a wide range of environmental
stresses. The regulatory function of ppGpp lies in the fact that it redirects transcription to overcome
starvation (Magnusson
et al
., 2005; Braeken
et al
., 2006). Studies on bacteria revealed that MazG gene
encodes enzymes that hydrolyze the harmful non-canonical nucleotides such as ppGpp (Galperin
et
al
., 2006). Thus these enzymes have been designated as “house-cleaning” enzymes. Other functions
of MazG are (i) it is implicated as a regulator of programmed cell death in
E
.
coli
and (ii) it prevents
the accumulation of ppGpp in response to amino acid starvation (Gross
et al
., 2006). It has been
suggested that the phage encoded MazG operates to reduce the pool of ppGpp in
Synechococcus
cells
(Clokie and Mann, 2006). Mann
et al
. (2005) sequenced the cyanophage S-PM2 genome and identifi ed
239 ORFs. Of these, ORF136 encodes a homologue of the bacterial MazG. The probability of MazG
altering the physiology of phage infected cells has been tested by Bryan
et
al
. (2008). By designing
primers internally to the ORF136 gene (a total length of 438 bp), PCR amplifi cation from the genome
of cyanophage S-PM2 resulted in a product of 279 bp (64% of the gene sequence). By employing
these primers, the genomes of 41 marine cyanophages (isolated from different marine waters) have
been amplifi ed by PCR. After conducting the sequence analysis of this product, computational
analysis of data was performed by BlastN and protein:protein searches. The sequences of 15 of the
cyanophages (S-Bn1, S-IO41, IO50, S-BP3, S-MM1, S-MM5, RS-9, RS-11, RS18, RS23, RS26, RS-27,
RS37, RS56, FWP) showed 99% identity to the reference phage S-PM2. A phylogenetic analysis of the
16 phage sequences (15 + reference phage S-PM2) with all known bacterial sequences included in
Clade A, Clade B and other sequences available in database of NCBI revealed that all 16 cyanophages
are closeted into a single group, so classifi ed as Group I. The translated sequences also exhibited
a high degree of similarity to bacterial MazG protein from the NCBI database. Group II consisted
of 26 cyanophages of which only 9 showed (IO15, IO18, IO17, IO39, IO41, RS38, RS-39, RS-85,
S-IO80) similarity to the MazG sequence of S-PM2. MazG gene is highly conserved in
Synechococcus
