CYANOPHAGES - Cyanobacteria - page 429

the invagination occurred fi rst at 3 h after infection and was distinct by the 4th h corresponding
to the end of eclipse period. By the end of latent period almost all the cells showed invaginated
lamellae and viriogenic stroma containing phage particles. The breakdown of host DNA has also
been noted in LPP-1 infected cells of P . boryanum coinciding with LPP-1 synthesis (Sherman and
Haselkorn, 1970b). A characterization of virus-induced proteins and the follow up of the course of
their synthesis during LPP-1 infection in the cells of P . boryanum has been reported (Sherman and
Haselkorn, 1970c). These workers identifi ed 3 classes of viral proteins: three proteins belonging to
the earliest class begin to appear soon after infection and their synthesis completed by 4th h. This
is followed by the synthesis of early proteins (another 3 proteins) by the 2nd h and continues to be
synthesized until lysis. The late class of proteins belonged to viral structural proteins. A total of 17
distinct virus induced proteins were detected after infection. They accounted for 65% of the coding
capacity of viral genome. Sherman et al . (1976) conducted detailed investigations on the ultrastructure
of infection and assembly of phage AS-1M. The fi rst noticeable evidence of phage infection was a
degradation of the nucleoplasm of the host cell. Precisely after 6 h of infection many phage precursors
in the process of assembly could be seen. Based on these studies, they have speculated that empty
phage heads are formed fi rst and then fi lled with DNA. Three classes of proteins are synthesized
after AS-1M phage infection in the cells of S . cedrorum . These are classifi ed as early proteins which
are produced until 4 h after infection, middle proteins which are produced until 3 h after infection
until lysis; and late proteins, i.e. the phage structural proteins which are synthesized until 4 h after
infection (Sherman and Pauw, 1976). They further suggested that a breakdown of the host DNA
begins immediately after infection that is optimal by 3 h. This process is in response to a phage-
dependent nuclease activity that has the characteristics of an early protein. Phage DNA synthesis
begins at about 3 h after infection and the enzymes responsible for this appear to be middle proteins
and require post-infection RNA transcription.
Transmission electron micrographs of cyanophage SM-2 infected cells of S . elongatus revealed
virion heads 10 h after infection correlating well with 6-8 h eclipse period for the virus (Leach
et al ., 1980). One of the signifi cant features of the development of the cyanophage SM-2 is that
photosynthetic lamellae remain intact throughout the infection cycle even upon accumulation of
additional virion heads.
The latent period of cyanophages of LPP-1 group has been detected to be 8 h, 6-7 h and 40 h
in LPP-1, LPP-1G and D-1, respectively (Safferman and Morris, 1964a; Padan et al ., 1967; Daft et al .,
1970). Similar to LPP cyanophages, N-1 has a latent period of 7 h (Adolph and Haselkorn, 1972). The
burst size of N-1 is 100 PFU/infected cell. AS-1 has a latent period of 8.5 h and an average burst size
of 50 PFU/infected cell (Safferman et al ., 1972). SM-2 has a latent period of 30 h with a rise period
of 20 h and an average burst size of 100 PFU/infected cell (Mackenzie and Haselkorn, 1972b). The
latent period of AC-1 is reported to be 30 h (Sharma and Venkataraman, 1977).
i) Dependency of replication cycle on photosynthesis : LPP-1 infected cells of P. boryanum showed
an inhibition of photoassimilation of carbon dioxide (Ginzberg et al ., 1968). The dependeny of LPP-
1G viral multiplication on photosynthesis is limited to the fi rst 3 h of eclipse period and for ATP
production (Padan et al ., 1970). In the presence of CCCP (inhibitor of cyclic photophosphorylation),
no virus is produced whereas in the presence of DCMU and anaerobiosis (presence of cyclic
photophosphorylation) the viral reproductive cycle is completed. Thus ATP for the reproductive
cycle is provided by photosystem I (PSI) through cyclic photophosphorylation. Wu et al . (1968)
demonstrated that energy for viral development is provided by either oxidative phosphorylation
or photophosphorylation and when both systems are operative, infection is faster than with either
system alone. In the absence of both systems, development is completely inhibited.