Biology Reference
In-Depth Information
responsible for increased rate of G.C to T.A transversions (Rocap
et al
., 2003). A comparison of the
genomes of strains of
P
.
marinus
strain MED4 and SS120 revealed that besides the absence of
ada
gene (that encodes 6-O-methylguanine-DNA methyltransferase which repairs alkylated forms of
guanine and thymine in DNA), a number of genes such as
mutY
(that encodes A/G specifi c DNA
glycosylase),
recQ
(that encodes Superfamily II DNA helicase),
recJ
(that encodes single-stranded
DNA-specifi c exonuclease),
exol
/
lxseA
(that is responsible for exonuclease VII large subunit) and
xseB
(that encodes exonuclease VII small subunit) are absent in MED4 whereas SS120 possesses
mutY
,
recJ
,
exol
/
lxseA
and
xseB
. On the contrary, the presence of all these genes including that of
ada
has been demonstrated in
Synechococcus
strains MIT9313 and WH8102 (Dufresne
et al
., 2005).
Heavy duplication of genes related to DNA repair and recombination (primarily
recA
) seems to be a
characteristic feature of the genome of
A
.
marina
MBIC11017. The percentage of such portions in the
genome seems to be higher (18.7%) than noted in case of
Synechocystis
sp. strain PCC 6803 (11.2%)
and
Anabaena
sp. strain PCC 7120 (5.8%) (Swingley
et al
., 2008).
8) DNA restriction and modifi cation
:
In
M.
aeruginosa
PCC 7806 there are 21 potential genes for
restriction enzymes localized together where as
M
.
aeruginosa
NIES-843 has 17 genes for restriction
enzymes. Of these, 14 genes are common to both these strains. Among these, seven and eight of
the restriction enzymes seem to be specifi c for
M
.
aeruginosa
PCC 7806 and
M
.
aeruginosa
NIES-843,
respectively. A comparison of the genomes of these two
Microcystis
strains with
C
.
watsonii
WH8501
and
Synechocystis
sp. strain PCC 6803 revealed that the presence of 6-mer sequences signifi es the
existence of restriction sites. As in case of
Synechocystis
sp. strain PCC 6803 the absence of 6-mer
sequences corresponded with the absence of restriction enzymes. A large majority of these 6-mer
sequences constitute palindromic sequences amounting to nearly 51% of the rarest 1% 6-mers in
M
.
aeruginosa
PCC 7806 (Frangeul
et al
., 2008). The genome of
M
.
aeruginosa
NIES-843 has 62 putative
restriction-modifi cation genes belonging to Type I RMs (4) and Type II RMs (58). In addition, four
potential RM-related loci are disrupted by insertion sequences (ISs) (Kaneko
et al
., 2007).
9) Transport and binding proteins
:
In the genome of
N
.
punctiforme
ATCC 29133 there are 89 ORFs
that encode ATPase domain of assigned and unassigned membrane-associated ATP-binding cassette
transport systems (designated accordingly as ABC-transporters). Additionally, 48 permeases not
associated with ABC-transporters for the transport of organic carbon and iron are present. Two
complete ATP-dependent phosphate transport systems (each consisting of
pstS
,
pstC
,
pstA
,
pstB
) and a
periplasmic protein for sulphate transport are present (Meeks
et al
., 2001). A gene cluster comprising
of
nirA
(nitrite reductase) -
nrtA
-
nrtB
-
nrtC
-
nrtD
(ABC transporter)—
narB
(nitrate reductase) is
present in
Anabaena
sp. strain PCC 7120 for the uptake and reduction of nitrite and nitrate, respectively.
However, there appear to be two independent nitrite/nitrate transport systems in
N
.
punctiforme
ATCC 29133. This is evident by the presence of
nirA
and
narB
with an intervening permease that is
meant for the transport of nitrite/nitrate. Besides these, a cluster of four genes bearing 90% similarity
to nrtABCD nitrate transporter of
Anabaena
sp. strain PCC 7120.
P
.
marinus
MIT9313 has lost a 25-gene
cluster that governs nitrate/nitrite transporter and nitrate reductase but nitrite reductase gene has
been retained. This gene is fl anked by a proteobacterial type nitrite transporter rather than a typical
cyanobacterial type nitrate/nitrite permease. This suggests that these genes have been acquired
through LGT which refl ects that these two strains are able to adjust to the particular environmental
niche in which the specifi c nutrient is prevalent. Accordingly,
P
.
marinus
MED4 that has even lost
the ability to utilize nitrite is adapted to grow at the surface waters where ammonia is available
and high light conditions prevail. On the other hand,
P
.
marinus
MIT9313 which can utilize nitrite is
adapted to grow at greater depths where nitrite is available and low light conditions prevail. Genes
