Biology Reference
In-Depth Information
phage is the presence of a “horn” which is a slender elongated fi brous protrusion of approximately
50 nm long and 10 nm wide, tapering to 2-5 nm at the tip. TEM of infected cells revealed the
eclipse period to be of 1 h. By 2 h, 30% of the infected cells showed fi ve or more phage heads in
the cell. Cell lysis ensued after 10 h of infection. The genome of Syn5 is 46, 214 bp long with a
237 bp terminal direct repeat. There are 61 ORFs and some resemble T7 phage gene sequences
and so Syn5 shares some of the features of T7. The existence of an integrase gene signifi es that the
phage is a temperate one. The structural proteins of the phage are encoded by eight of the 11 ORFs
assigned to structural proteins. The amino acid sequence of these proteins has similarity to the
proteins of enteric phages. The occurrence of genes is without identifi able promoter regions and
Shine-Dalgarno sequences. The presence of overlapping start and stop codons signifi es a translational
coupling by which the translation of the downstream genes is dependent on the translation of the
preceding genes as has been identifi ed in T4. As per these criteria, thirty fi ve of the Syn5 ORFs could
be translationally coupled. The arrangement of genes in Syn5 is very similar to that found in other
cyanophages such as P60 and P-SSP7. PSI and PSII reaction center genes are absent in the genome
of Syn5 such as those reported in other cyanophages (Mann
et al
., 2003; Sullivan
et al
., 2005; Weigele
et al
., 2007; Sharon
et al
., 2009). Raytcheva
et al
. (2010) reported an eclipse period of ~45 min and a
latent period of ~60 min with a burst size of 20-30 particles per cell, in contrast to the fi ndings of
Pope
et al
. (2007). The intracellular assembly of Syn5, as observed by them through cryo-electron
images, showed the presence of protein capsids fi lled with scaffolding protein, though this protein
is not associated with mature free virion particles. According to them the presence of the scaffolding
protein in virion heads even before packaging of DNA into the heads is akin to the intracellular
development of enteric dsDNA phages. Since cyanobacteria are the earliest prokaryotes this type of
intracellular development of cyanophages seems to have originated with these organisms.
e)
Cyanophage Pf-WMP4:
This phage infects
Phormidium foveolarum
and resembles T7. The genome
of this phage is 40,938 bp long and consists of 45 ORFs, nine of which are similar to T7.
In vitro
transcription led to the identifi cation of seven promoters to the left (Liu
et al
., 2007).
f)
Cyanophage Ma-LMM01
: The phage genome is 162,109 bp long with 184 ORFs. Database search
revealed that there are 44 (24%) ORFs whose functional identity could be assigned and the rest of
the 140 ORFs have no detectable homologues. The most signifi cant feature is the presence of
nblA
gene that mediates degradation of phycobilisome complex. A site-specifi c recombinase or integrase
(
int
- gene used by temperate phages) and two prophage anti-repressor genes (that mediate integration
into host genome) are located within 7.5 kb with a typically low G+C content. Similar to the other
cyanomyoviruses or phage T4, Ma-LMM01 genome has 4.3% and 5.4% of transmembrane proteins
and proteins with signal peptides, respectively. Only four ORFs-ORF1, ORF41, ORF108 and ORF109
had their best sequence homologies in other viruses. The virion head consists of two major head
proteins resembling phage lambda. ORF19 encodes tail sheath protein (Yoshida
et al
., 2008).
g)
Cyanophage syn9
: This cyanophage has a contractile tail but this has been treated as an unclassifi ed
virus (Yoshida
et al
., 2008). The genome of this phage is 177,300 bp long with 226 putative ORFs and
six tRNA genes. The phage has 19% of the genes that bear resemblance to phage T4. The assembly
of capsid proteins resembles those of and bears structural and topological relationships with phage
SPO1 and herpes virus. Evidences for different patterns of DNA replication have been presented
(Weigele
et al
., 2007).
h)
P-SS2
: This is the fi rst marine cyanosiphovirus whose genome has been sequenced. This
phage has been isolated against
P. marinus
MIT9313 as the sensitive host from 83 m deep
