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genome contains as many as 1971 genes representing clusters of atypical genes (CAGs) purported
to have been acquired through LGT. There are 838 genes (16%) whose products are not similar to
>40% of any proteins in M . aeruginosa NIES-843 and of these 644 of the proteins bear no apparent
similarity to >40% of any proteins in the other cyanobacterial genomes sequenced so far. The
putative transposases are encoded by 362 genes (representing 6.8%). The proportion of large repeats
equals the number in M . aeruginosa NIES-843 (i.e. 11.7%) and the length of the repeats is >1000
bases and there is >90% identity in these sequences. There are two rRNA operons and 41tRNA loci
on the chromosome. Three groups of proteins have been identifi ed after BLASTp analysis of 5,292
genes, on the basis of a sequence identity of 40% between individual protein sequences. The fi rst
group designated as “Maeru40” group consists of 764 genes (14.4%) specifi c to M. aeruginosa strain
PCC 7806 and not found in 15 other cyanobacterial genomes under comparison. Of these, 438
genes (8.3%) have no homologues in the databases. The second group (“core40” group) comprises
652 proteins (12.3%) that had high degree of similarity with at least one gene sequence in each of
the 15 other genomes. The third group (“other40”) has the rest of the genes bearing high degree of
similarity to the coding sequences in the other genomes. Signifi cantly, the existence of two genes, the
fi rst one ( mic5251 ) that is similar to a gene sequence of Synechocystis sp. strain PCC 6803 responsible
for perceiving osmotic stress and cold stress and the second one ( mic5237 ) identical to Anabaena sp.
strain PCC 7120 gene known as orrA gene help M . aeruginosa strain PCC 7806 to overcome stress due
to lower temperature and high pressure of waters during winter months. Gene sequences bearing
homology to eukaryotic genes actM (that encodes actin) and pfnM (that synthesizes actin cognate
binding partner) have been discovered in this strain that enable it to resist hydrostatic and osmotic
pressures. Cell wall and polysaccharide synthesizing genes that have a role in colony formation have
been identifi ed in this strain as also a cluster of 12 genes responsible for the formation of gas vesicles.
M . aeruginosa PCC 7806 appeared to be closer to Cyanothece sp. ATCC 51142, Cyanothece sp. CCY0110
and C . watsonii WH8501 on the basis of comparison of coding sequences of 44 cyanobacterial genomes.
On the other hand, on the basis of 16S-23S rDNA phylogeny M . aeruginosa PCC 7806 appeared closer
to Synechocystis sp. strain PCC 6803. The number of orthologous genes shared among these three
closely related cyanobacteria with M . aeruginosa PCC 7806 is 1789. Although there appeared to be a
low synteny score in the cyanobacterial genomes when compared to bacterial genomes there are at
least four gene clusters relating to ribosomal proteins that are syntenic in the genomes of M . aeruginosa
strain PCC 7806, C . watsonii WH8501 and Synechocystis sp. strain PCC 6803. Phosphate transport
system (6 genes), inorganic carbon concentrating mechanism (5 genes), ATP synthase (8 genes) and
genes of unkown function (4 genes) are the other gene clusters identifi ed to be syntenic.
v) Genomes of Prochlorococcus strains : Rocap et al . (2003) sequenced and compared the genomes
of HL-adapted P. marinus MED4 and LL-adapted P. marinus MIT9313. The genome of the former is
the smallest (with 1,657,990 bp and a G+C content of 30.8%) among oxygenic photoautotrophs while
the genome of the latter is signifi cantly larger (with 2,410,873 bp with a G+C content of 50.7%). The
number of protein-coding genes and those proteins with assigned function are 1,716 and 1,134 in
MED4 and 2,278 and 1,366 in MIT9313, respectively. There are 1352 genes common to both strains.
Interestingly, excepting 38 of the common genes the rest all are shared by Synechococcus sp. strain
WH8102. Genes responsible for the synthesis of chlorophyll a/b binding protein, (divinyl), chlorophyll b
(i.e. chlorophyll a oxygenase that can convert divinyl chlorophyll a into divinyl chlorophyll b ) and
α-carotene (lycopene-ε-cyclase) are characteristic of the genomes of Prochlorococcus. The genome of
MED4 has 364 genes specifi c to it and the corresponding genes are absent in MIT9313. Likewise,
MIT9313 has 923 genes specifi c to it but these are absent in MED4 genome. The smaller size of MED4
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