CYANOPHAGES - Cyanobacteria - page 431

in Synechococcus sp. strain PCC 6301 cells that is comparablable to DCMU-induced protection of D1
protein turnover, in AS-1 infected cells (Teklemariam et al ., 1990).
ii) Nucleic acid metabolism : A ribonuclease for cyanophage AS-1 has been detected in the infected
cells of A . nidulans . Further purifi cation and its characterization have shown a relative specifi city
of uridylic acid (Lehmann et al ., 1979). Cyanophage AS-1 specifi c endonuclease has been purifi ed
(molecular weight 40,000 ± 4000) from A . nidulans and it is further reported to bring about breakdown
of host DNA (Szekeres, 1981).
The DNA metabolism of cyanophage AS-1 infected cells of A . nidulans has been investigated
in detail. Hwang-Lee et al . (1982a) identifi ed an unique species of phage-induced light DNA (PIL-
DNA) in the infected cells accounting for as much as 50% of total DNA at ~5 h post-infected period
and the same was found in negligible amounts after 9 h during a 16 h infective cycle. This has been
further confi rmed by a differential labelling experiment in which C 14 -incorporation begins around
2 h of post-infection and is maximum by 4-6 h followed by a decrease thereafter. Whereas H 3 -
adenine accumulates preferentially in AS-1 DNA by the middle of the lytic cycle at which time no
H 3 -adenine incorporation took place in PIL-DNA (Hwang-Lee et al ., 1982b). The observation that
PIL-DNA could be differentially labelled has further been extended to other phage-host systems,
i.e. AS-1, AS-1M, AS-1 lab stock with A . nidulans 625 and S . cedrorum 1191 by using precursors, i.e.
H 3 -uracil, H 3 -thymidine and C 14 -glucose-6-phosphate (Blashka et al ., 1983).
Hwang-Lee et al . (1983a) demonstrated that both phage and host DNA are involved in the
production of PIL-DNA and its production is more dependent on the integrity of the phage DNA than
that of the host. Subsequently, biophysical characterization of PIL-DNA revealed that it is modifi ed
by the presence of 5-methylcytosine and resembles AS-1 DNA in nucleotide sequence (Hwang-Lee
et al ., 1983b). Bancroft and Smith (1988a,b) divided cyanophages A-4L, AN-13, AN-23, A-1L and
AN-10 into two classes on the basis of analysis of restriction endonuclease sites. Cyanophages A1-L
and AN-10 possess adenosine methylated GATC sequences (as Sau3AI cleaves but MboI does not)
and are thus tolerant to this dam-like methylation. It is interesting to note that DNA of the host
Anabaena sp. strain PCC 7120 also appears to be modifi ed at the same base sequences. But AN-10
showed considerably less modifi cation. The rest of the three cyanophages neither contained adenosine
methylated or unmodifi ed GATC sequences (as both Sau 3AI and MboI fail to cleave) and so are
intolerant to such methylation. These results suggest that DNA methylation as well as restriction may
be detrimental for phage reproduction if phage DNA is correspondingly not methylated. Cyanophage
A-4(L) represents an extreme example of sequence counter-selection as it has eliminated from its
genome many or all of the numerous sequences modifi ed in Anabaena DNA (Lambert and Carr, 1984;
Bancroft and Smith, 1988a,b). Bancroft and Smith (1989) further mapped the average sites of several
restriction endonucleases in DNA isolated from fi ve cyanophages, i.e. A-1(L), A-4(L), AN-10, AN-13
and AN-23 which were replicated in Anabaena sp. strain PCC 7120. It is appropriate to mention here
that the cyanophages A-1(L) and A-4(L) were isolated by Koz'yakov (1977) and the rest three were
isolated by Hu et al . (1981). AN-10, though isolated by Hu et al . (1981) as a contaminant from A-4L,
differed in the restriction map but appeared to be related to A-1(L). On the basis of restriction maps
the cyanophages AN-13 and AN-23 are related. These observations are further supported by the fact
that the genomes of these two phages are 46 kbp long and hybridization between their DNAs has
also been possible (Hu et al ., 1981). The restriction maps of A-1L and AN-10 appeared to be circular
while the maps of other phages are linear.