CYANOPHAGES - Cyanobacteria - page 422

SS represents Sargasso Sea from which collection was made and M represents a myovirus and
2 represents the second virus in the series. Safferman et al . (1983) proposed a classifi cation for
cyanophages in accordance with ICTV guidelines for bacteriphage classifi cation. The cyanophages
whose morphology and ultrastructure have been described till then have been included and classifi ed
into three families, i.e. Myoviridae, Siphoviridae (or Styloviridae) and Podoviridae and recognized
representative genera of cyanophages as Cyanomyovirus, Cyanosiphovirus and Cyanopodovirus,
respectively. A cyanomyovirus is a cyanophage with contractile ('myos' in Greek means muscle)
tail. AS-1 is recognized as the type species. Other cyanophages such as AS-1M, N-1, A-1(L) and A-2
having similar morphology are included here. Likewise, Cyanosiphovirus is a cyanophage with
non-contractile tail but the tail extends as a pillar ('sipho' in Greek means a 'siphon' or tube-like).
The type species is S-1. Other members are S-2L and SM-2. The cyanopodovirus is characterized by
the presence of a short tail that is smaller than the length of the head. Because this short tail assumes
the shape of a foot ('podos' in Greek means foot) it is named as a cyanopodovirus. LPP-1 is the type
species. Other members are LPP-1G, LPP-2, SM-1, A-4(L) and AC-1.
II. FRESHWATER CYANOMYOVIRUSES
The type species of cyanomyoviruses AS-1 that infects Anacystis nidulans and Synechococcus cedrorum
has been isolated from a waste stabilization pond located at Sun City, Lake Centre, Florida (Safferman
et al ., 1972). The AS-1 particle contains dsDNA and appears polyhedral in structure with a long
contractile tail. The edge to edge diameter of its head is 90 nm and the tail measures 243.5 nm
long and 22.5 nm in diameter. Magnesium ions are not required for stability of virus, while NaCl
increases the adsorption rate by 34%. The virus has greatest stability between pH 4 and 10. The
other cyanomyoviruses isolated and characterized were AS-1M, A-2, A-1(L), Ma-LMM01, N-1 and
S-3L. Cyanophage AS-1M is morphologically identical to AS-1 in having edge-edge diameter of
the head of 90 nm and a contractile tail of 240 nm long and 22.0 nm in diameter. It also infects A .
nidulans and S . cedrorum but exhibits rapid multiplication cycle (8 h) than AS-1 and completely lyses
the host within 12 h after infection (Sherman and Connelly, 1976). Cyanophage A-2 active against
Anabaena variabilis (Granhall and Hofsten, 1969) was reported from Sweden. A-1(L) through A-9
(L) series of cyanophages have been isolated from Leningrad (Kozyakov, 1977). A-1(L) and A-4(L)
which sensitize A . variabilis have been examined in detail and the rest of them have been lost (Hu et
al ., 1981; Gromov, 1983). The diameter of the capsid of A-1(L) is 60 nm and has a contractile tail of
83 nm long. Other cyanomyoviruses described infect various strains of Anabaena and Nostoc among
which AN-10 and AN-15 are morphologically alike with phage heads (60 and 63 nm, respectively)
and contractile tails (100 nm; Hu et al ., 1981).
A cyanophage (Ma-LMM01), a myovirus with a head (86 nm dia.) and tail (209 nm length and 24
nm width) possessing a central tube of 9 nm, has been isolated from Lake Mikata in Fukui Prefecture,
Japan that infects the toxic strain of Microcystis aeruginosa NIES298 (Yoshida et al ., 2006). This cyanophage
with a latent period of 6 to 12 h has a burst size of 50 to 120 particles per cell. The phage possesses
dsDNA as its genome with a size of ca. 160 kbp and a low level of methylation. Phylogenetic analysis
of the deduced amino acid sequence of the putative sheath protein revealed that it forms a sister group
with freshwater and marine cyanophages although it possesses morphological similarities with T4
like-phages. In view of the fact that M . aeruginosa NRC-1 that was used as a host strain for the isolation
of other cyanophages (SM-1 by Safferman et al ., 1969; SM-2 by Fox et al ., 1976) was later shown to be a
strain of Synechococcus sp. (Suttle, 2000), the report by Yoshida et al . (2006) assumes siginifi cance and
seems to constitute the fi rst report on the isolation of a cyanophage infectious against a toxic strain