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total of 3168 potential protein-coding genes were allocated on the chromosome. But in a subsequent
analysis conducted in 2002, the assignment of potential protein-coding regions has been revised
to 3264 (http://www.kazusa.or.jp/cyanobase/synechocystis/). Of the allocated genes, 145 (4.6%)
have already been reported. A sub-total of genes similar to genes of known function are 1623 and
similar hypothetical genes are 1133. These two together form genes similar to registered genes and
the sub-total is 2,756. The genes without similarity number up to 508. The essential coding capacity
of the genome of this organism is 55% and the remaining 45% (1426 genes) showed no signifi cant
similarity to any registered genes. The overall gene density (described as the number of genes in
relation to length of bases) has been found to be approximately one gene per 1.1 kb. The average
length of encoded gene product has a length of 326 amino acids, the longest one being 4,199 amino
acids long (
Slr 408
). Two copies of rRNA gene clusters with identical 5,028 bp sequences are present.
However, these are arranged in reverse orientation with respect to one another at map positions 68% and
93% of the genome. There are 42 tRNA genes in the genome of
Synechocystis
sp. strain PCC 6803.
As many as seven plasmids are reported in this organism. Three of them are small plasmids
(with 5.2 kb, 2.4 kb and 2.3 kb length) and the rest are large. The nucleotide sequencing of four large
plasmids (pSYSM:120 kb; pSYSX:106 kb; pSYSA:103 kb and pSYSG:44 kb) of
Synechocystis
sp. strain
PCC 6803 revealed a total of 397 potential protein-coding genes and 77% of these have unknown
function. The presence of sequence HIP1: 5'-GCCATCGC-3'specifi c in these plasmids indicates that
they may have originated from different cyanobacterial strains. Of the 397 genes, similarity search
for proteins coded by these genes indicated 23% (92 genes) having sequence similarity to genes of
known function, 36% of genes (142 genes) being identical to hypothetical genes and the remaining
41% (163 genes) were non-identical to genes of known function (Kaneko
et al
., 2003).
ix) Genome of T. elongatus BP-1
:
The genome of this thermophilic cyanobacterium is a circular
molecule consisting of 2,593,857 bp with an average G+C content of 53.9% (Nakamura
et al
., 2002).
No plasmid sequences have been encountered. The potential protein-coding sequences are 2,743 of
which total number of protein-coding sequences is 2,475. Of these, 1,569 (i.e. 63%) genes have matched
with the gene sequences of
Synechocystis
sp. strain PCC 6803 and
Anabaena
sp. strain PCC 7120. Genes
of unknown function are 522 in number.
T
.
elongatus
BP-1 genome shares 116 (3%) and 254 (10%)
genes with
Synechocystis
sp. strain PCC 6803 and
Anabaena
sp. strain PCC 7120 genomes, respectively.
The gene density is one gene per every 1,048 bp. The start codons identifi ed are ATG, GTG, TTG or
ATT. There is one copy of rRNA gene cluster in the order of 16S-23S-5S at the coordinates 2,330,963-
2,336,733. There is a group I intron sequence of 745 bp length in the 23S RNA gene cluster that encodes
a homing endonuclease. An eight base palindromic sequence known as highly iterated palindromic
sequence 1 (HIP1) is represented by 3,681 copies in the genome of this cyanobacterium. The frequency
of occurrence of HIP1 is much higher (one copy per 705 bp) than found in
Synechocystis
sp. strain PCC
6803 (one copy per 1131 bp) and
Anabaena
sp.strain PCC 7120 (one copy per 1219 bp). Moreover, the
frequency of occurrence of HIP1 in the genome of
T
.
elongatus
BP-1 itself varied as it was highest in
protein-coding regions (one copy per 672 bp), lower in intergenic regions (one copy per 1209 bp) and
lowest in RNA-encoding regions (one copy per 4328 bp). On the contrary, only one copy of HIP1 has
been encountered in 110,339 bp regions corresponding to mobile introns and insertion sequences. There
are 42 tRNA genes representing 42 tRNA species widely distributed all over the genome.
x) Genomes of Arthrospira
:
(a) A. (Spirulina) platensis NIES-39:
Fujisawa
et al
. (2010) reported the
complete genome sequence
A
.
platensis
NIES-39 based on optical mapping. A circular chromosome
(of 6.8 Mb) possesses 6630 protein-coding sequences of which 5157 are orthologues (78%) of known
function. A comparison of 38 cyanobacterial genomes brought out 694 COGs common to all six groups
