GENOMES - Cyanobacteria - page 91

There are two rRNA operons on the circular chromosome with none being represented on the linear
chromosome. There are 43 tRNA genes on the circular chromosome. When compared to the circular
chromosome which has 45.7% genes with no assigned function, the linear chromosome has a higher
percentage (71.7%) of genes of unknown function. At least 50 of the protein-coding genes on the
linear chromosome are functional. The linear chromosome has 38 genes whose corresponding copies
are represented in the circular chromosome and another 42 genes on linear chromosome are unique
that have homologues in other organisms. The length of the largest plasmid is 39,620 bp while the
medium ones have 31,856 bp and 14,685 bp and the smallest one has 10,244 bp.
iii) Genome of G. violaceus PCC 7421 : Complete genome structure of G . violaceus PCC 7421 has been
presented by Nakamura et al . (2003) and the details of individual genes are listed in Cyanobase at
http://www.kazusa or jp./cyanobase. This cyanobacterium is distinctive in its cellular organization
as it lacks photosynthetic thylakoids. The genome of this organism is a circular molecule of
4,659,019 bp with an average G+C content of 62%. No plasmids have been detected during this study.
The number of potential protein-coding genes is 4,430. Of these 41% (1836 genes) showed sequence
similarity to genes of known function and 37% (1635 genes) of them are hypothetical genes. The rest
22% (959 genes) do not bear any resemblance to the known and registered gene sequences at public
databases. There is one copy of rRNA gene cluster in counter-clockwise direction in the order of 16S-
trnI-trnA-23S-5S. A total of 45 tRNA genes (including one rRNA gene cluster) are present representing
44 tRNA species. G . violaceus is unique in possessing an intron-less trnL-UAA. A single gene for
tm-(transfer-messenger) RNA is found that probably plays a role in the degradation of aberrantly
synthesized proteins. It is interesting to note that genes showing sequence similarity to those of
E. coli and Streptomyces coelicolor are present. After substraction of these genes, G . violaceus PCC 7421
possesses 610 genes unique to three other cyanobacteria ( Anabaena sp. strain PCC 7120, Synechocystis
sp. strain PCC 6803 and T . elongatus BP-1) and at least half of them are of known function (related to
antenna components, chlorophyll synthesis, photosystems and carbon fi xation). At least 995 genes of
this cyanobacterium (23%) do not bear any resemblance to three genomes under comparison
(see Table 5). Of these, 684 do not bear any resemblance to the registered genes.
iv) Genome of M. aeruginosa : The genomes of two strains of this cyanobacterium M . aeruginosa
NIES-843 and M . aeruginosa PCC 7806 have now been sequenced.
a) Genome of M. aeruginosa NIES-843 : Kaneko et al . (2007) sequenced the genome of M . aeruginosa
NIES-843. A single circular chromosome of 5,842,795 bp long with an average G+C content of
42.3% is present. The putative protein-coding genes are 6,312. Of these 45% of them show sequence
similarity to genes of known function, 32% are similar to hypothetical genes and the remaining
23% bear no resemblance to any known genes in the databases. One unique feature is the presence
of putative transposases (469 gene sequences amounting to 7.4%) and large repeats (11.7% of the
genome). Genes involved in the biosynthesis of microcystins, cyanopeptolin and for modifi cation of
small polypeptides have been identifi ed. A large number of genes belong to restriction modifi cation
system. There are two operons for rRNA synthesis and 42 tRNA genes representing 41 tRNA species.
An 8 bp palindromic sequence (5'-GCCATCGC-3'), characteristic of many cyanobacterial genomes,
known as HIP1 is present in 1821 copies in all in the genome of NIES-843 and the frequency of its
occurrence is one copy/3209 bp which is quite lower than in others .
b) Genome of M. aeruginosa PCC 7806 : It has a circular chromosome of 5,172,804 bp long with a
G+C content of 42%. The putative protein-coding sequences are 5,292 (Frangeul et al ., 2008). The
proteins of known function are found to be 3,322 (amounting to 63% of the coding capacity). The