Biology Reference
In-Depth Information
The number of genes for heterocyst development has been found to be 58 and 54 in
C
.
raciborskii
CS-505 and
R
.
brookii
D9, respectively. Though only four genes make a difference in the number of
genes required for heterocyst development,
R
.
brookii
D9 does not form heterocysts. In
Anabaena
sp. strain PCC 7120 the number of genes required for heterocyst development have been found to
be 77, of which 55 homologues are represented in
C
.
raciborskii
CS-505 without the gene sequences
for
hetC
,
ccbP
,
hetL
,
hetN
and
patS
. On the other hand,
R.
brookii
D9 lacks genes for nitrogen fi xation
(
nif
cluster-related genes, uptake hydrogenase gene cluster with
hupSL
and
hupW
and a set of genes
involved in general metabolism) and heterocyst-specifi c glycolipid clusters (
hglEGDCA
and
hetM
).
However, it possesses identical cluster of genes required for heterocyst envelope polysaccharides
as present in
C
.
raciborskii
CS-505. The COGs identifi ed for
nif
gene cluster in the heterocystous
cyanobacteria on the basis of sequence comparisons is 49 which is the upper limit and 38 of them are
noted in
C
.
raciborskii
CS-505. All the
nif
genes are present together in a 15 kb region in the genome
of
C
.
raciborskii
CS-505 very much resembling the arrangement of genes in
nif2
gene cluster of
A
.
variabilis
ATCC 29413 (see for details Chapter 4). There is a greater degree of synteny in the genes
for biosynthesis of cylindrospermopsin and saxitoxin in both the organisms (Stucken
et al
., 2010).
xiv) Genome of Anabaena
sp.
strain PCC 7120
:
The complete genomic sequence of
Anabaena
sp. strain
PCC 7120 was reported by Kaneko
et al
. (2001). A single circular chromosome of 6,413,771 bp long
with a G+C content of 41.3% is present in this organism. The total number of protein-coding genes
is 5,368. The genome has a coding capacity of 45% and the encoded proteins showed similarity to
known and predicted proteins of known function. Hypothetical gene products amount to 27% and
the remaining (28%) genes lacked similarity to genes of known and predicted proteins in public
databases. There are four sets of rRNA operons (rrnA-rrnD) in the sequence of 16S-23S-5S. The
number of tRNA genes is 48 and 42 tRNA genes correspond to 42 tRNAs. Genes
fdxN
,
nifD
and
hupL
present in vegetative cells encode 57, 12 and 10 proteins, respectively including recombinases that
help in excision and genome rearrangement during heterocyst differentiation.
Six plasmids designated as pCC7120α (with a length of 408,101 bp), pCC7120β (186,614 bp),
pCC7120γ (101,965 bp), pCC7120δ (55,414 bp), pCC7120ε (40,340 bp) and pCC7120ζ (5,584 bp) have
been characterized. From these plasmids, more than 100 genes have been cloned and sequenced
much earlier to the sequencing of the genome. The distribution and functional identity of some of
the genes is now known. A total of 764 genes have been assigned to the six plasmids. Excepting
pCC7120ζ, the rest of them show genes for site specifi c recombinases, two of these are present on
pCC 7120α while the others have one each. A gene (
all7280
) that encodes adenine-specifi c DNA
methyltransferase and. ζ-carotene desaturase (
all7255
) which converts ζ-carotene to lycopene are
located on plasmid pCC7120α. Of the 11 putative sigma factors three (
SigB
,
SigB3
and SigB4) have
been assigned to the plasmids. Genes for DNA replication (i.e., DNA polymerase III β subunit, DNA
polymerase γ and τ subunits, DNA polymerase δ' subunit and single-strand DNA-binding proteins)
are present on the plasmid pCC7120β. A cluster of three genes presumptively involved in encoding
ABC phosphonate transporter is present on pCC7120γ. Sugaya
et al
. (2003) investigated the probable
causes for the large genome size in
Anabaena
sp.strain PCC 7120 by comparing the gene-location
distance and the distribution patterns of functional categories in the genomes of
Anabaena
sp. strain
PCC 7120,
Synechocystis
sp. strain PCC 6803 and
T
.
elongatus
BP-1. They suggested that the present
Anabaena
sp. strain PCC 7120 genome might have resulted due to a whole genome duplication
event during evolution. Subsequently, they further ruled out the sequential tandem duplications