Biology Reference
In-Depth Information
events and has contributed to the evolution of these cyanomyoviruses. A bioinformatics analysis of
GOS expedition viral metagenomes in combination with the genomes of viruses and microorganisms
enabled Sharon
et al
. (2011) to identify the presence of hitherto undiscovered 34 microbial gene
families in the 452 viral scaffolds of the metagenome. These are related to energy metabolism (electron
transport genes), PSI and PSII, translation and post-translation related genes. This amply signifi es
that the marine viruses control translation and post-translational events in their host cells besides
the already known transcriptional control.
XII. LYSOGENY
The best example for explaining lysogenic behaviour is phage λ and
E
.
coli
system. This has been
used as a tool in gene clong experiments as well. A number of workers reported lysogenic behaviour
among cyanobacterial cells and their induction by various agents. However, the nature of temperate
phages, the molecular biology of their integration and excision events and the type of transductional
events mediated by these phages still remains obscure. The following account summarizes the nature
of lysogenic behaviour and of temperate cyanophages of both freshwater and marine origin.
i) Lysogeny in freshwater cyanophages
:
A number of workers have reported the lysogenic behavior
of
P
.
boryanum
cells consequent upon infection by LPP-1 and these reports focussed their attention
on a variety of conditions that are helpful in inducing a lytic cycle. Singh and Singh (1972) not only
observed lysogeny in
P
.
boryanum
but also demonstrated the transfer of antibiotic resistance marker
from resistant cells of
P.
boryanum
to the antibiotic-sensitive cells through transduction. Lysogeny was
for the fi rst time reported due to infection of LPP-1D (Delaware strain) and LPP-2 that are temperate
viruses on
P
.
boryanum
(Cannon
et al
., 1971). Cannon and Shane (1972) reported the effects of antibiotic
stress on protein synthesis in the establishment of lysogeny in
P
.
boryanum
cells. Padan
et al
. (1972)
isolated a temperate cyanophage LPP2-SPI which was able to enter into lysogenic cycle in
P
.
boryanum
.
Studies on stable lysogeny by SPI cts1, a ts-mutant of SPI and its induction by heat (treatment at
40°C) of host cells of
P
.
boryanum
revealed that shortly after infection a leakage of macromolecules
from the host cells was observed with a simultaneous inhibition in CO
2
fi xation. However, induced
prophage neither caused cellular leakage of macromolecules nor CO
2
fi xation was inhibited (Rimon
and Oppenheim, 1975). Cannon
et al
. (1976) cultured
P
.
boryanum
IU594 and
P
.
boryanum
-co-resistant
to LPP-1, LPP-1D and LPP-2 in chemostats to evaluate the development of phage-resistant strains and
the development of lysogeny. After three oscillations in the virus titer and cell numbers both of them
stabilized and due to development of LPP 1-resistant strains of
P
.
boryanum
the phage was eliminated.
However, the existence of phage titres of LPP-1D and LPP-2 throughout the growth in chemostat
signifi es the induction of lysogenic cells.
P
.
uncinatum
cells lysogenic for cyanophage LPP-1 produced
phage at the rate of 3 x 10
3
PFU ml
-1
and treatment with mitomycin C enhanced phage production
by 5-fold. Treatments with UV-light and heat did not lead to induction of lysogenic cells (Bisen
et
al
., 1986b). A temperate cyanopodovirus N(S)1 that formed turbid plaques on a species of
Anabaena
77S15 has been described (Franche, 1987). Sode
et al
. (1994) isolated a marine temperate cyanophage
ms-1 infecting
Synechococcus
sp. NKBGO42902, a strain that is known to produce cytokinin-like
compounds. The induction of the temperate cyanophage ms-1 in the lysogenic cells by treatment
with copper ions (3.1 x 10
-4
M of copper sulphate) appeared to be specifi c for Cu
2+
as other metallic
ions Cr, Pb, Co and Zn were not able to induce the lytic cycle. Another cyanobacterium that has been
reported to be lysogenic is
Phormidium persicinum
. The phage belongs to the family of Siphoviridae and
has a genome size of 50 kbp. Induction of lysogenic cells into lytic cycle in presence of mitomycin C
