GENOMES - Cyanobacteria - page 90

(G+C) in adjacent regions of chromosome from an arbitrary start to a given point in a sequence. The
value reaches its maximum at the termination point while it is at its minimum at replication origin.
Thus a GC skew analysis is conducted all over the genome to identify the probable shift to locate
the origin of replication of the chromosome (Grigoriev, 1998).
II. GENOMES SEQUENCED
A brief account of the basic features of 39 fully sequenced cyanobacterial genomes is presented here
followed by a comparative account of the major groups of functional categories of genes.
i) Genome of A . marina MBIC 11017 : A unicellular cyanobacterium was initially isolated as a minor
symbiont from a colonial ascidian together with a major symbiont Prochloron by Miyashita et al .
(1996) from Palau Island. The cells of this bacterium are spherical, ellipsoidal, sheathed, non-motile
and devoid of gas vacuoles. Its growth in ascidians has been confi rmed by the work of Miyashita
et al . (2003) and that of Kühl et al . (2005). It is also reported to grow in association with eukaryotic
macroalgae (Murakami et al ., 2004; Ohkubo et al ., 2006) and in microbial mats in the Salton Sea (a saline
and highly eutrophic California Lake; Miller et al ., 2005). A characteristic feature of this bacterium
is the presence of chlorophyll d (3-desvinyl-3-formyl chlorophyll a ) as the major light-harvesting
pigment which has a formyl group at 3rd position in ring A structure. The absorption maximum of
this pigment lies at 700 nm in organic solvents and up to 715 nm in vivo . The pigment composition
and the organization of photosynthetic reaction centers (Miyashita et al ., 1997; Akiyama et al ., 2001,
2002, 2004) have been determined and the taxonomic identifi cation as A . marina was subsequently
reported (Miyashita et al ., 2003). Swingley et al . (2008) reported a total genome size of 8.3 Mb that
includes the main genome of A . marina MBIC 11017 (with 6,503,723 bp) and 9 circular plasmids
(pREB1-pREB9) of variable sizes. The largest plasmid is pREB1 (with 374,161 bp) and the smallest
being pREB9 (with 2,133 bp). The G+C content of the main genome is 47% and that of the plasmids
ranged from 43% (pREB9) to 47% (pREB1). The total number of genes in the main circular genome is
6,342 of which 14 represent pseudogenes. The genome has a coding capacity of 85% and the average
length of a gene is 867 bp. There are six rRNA genes with 69 tRNAs. The ORFs without similarities to
known genes are 28% and the conserved hypothetical ORFs range up to 18%. The insertion elements
in the genome number up to 285 and their number in the plasmids varies from one (as in pREB9)
to 30 (as in pREB1). The number of ORFs in the plasmids is in decreasing order, i.e. 417, 392, 382,
and 279 in case of pREB1, pREB2, pREB3 and pREB4 respectively. Likewise in plasmids pREB5,
pREB6, pREB7, pREB8 and pREB9 there are 224, 192, 174, 120 and 4 ORFs respectively. Several of
the plasmids share signifi cant regions of homology and the genes encode key proteins governing
important metabolic reactions.
ii) Genome of Cyanothece sp. ATCC 51142 : The genome of this marine diazotrophic cyanobacterium
has been completely sequenced by Welsh et al. (2008). The most unique feature of this genome is the
existence of six separate elements, a circular chromosome, four plasmids and a linear chromosome.
The circular chromosome is 4,934,271 bp long with a G+C content of 37.9%. The linear chromosome
is 429,701 bp long with a G+C content of 38.6%. Put together the genome has a length of 5,460,377 bp
with a G+C content of 37.9%. The predicted genes in the circular and linear chromosome have been
determined to be 4,762 and 449, respectively making up a total of 5,304 genes in this organism. The
genes with assigned function are 2,584 and 127, respectively in the circular and linear chromosomes
making up a total of 2,735 genes (51.6%). The hypothetical genes and genes of unknown function
are 1,710 and 468 in the circular chromosome and 298 and 24 in the linear chromosome, respectively.