Biology Reference
In-Depth Information
(
Price et al., 2012
). However, it is important to bear in mind that the bio-
informatic method of 'prediction by analogy' has its limits. Until we do the
experiments, we may mispredict the real function or properties of a gene
product (
Bender, 2011
;
Domain, Houot, Chauvat, & Cassier-Chauvat, 2004
;
Figge, Cassier-Chauvat, Chauvat, & Cerff, 2001
).
The genome of cyanobacteria appeared to be widely diverse (
Table 5.1
),
probably as the results of gain and loss of genes transferred by cyanobacterial
plasmids, insertion sequences and cyanophages (see below in section 1.2).
Most cyanobacteria possess one circular chromosome with a size ranging
from about 1.44 Mb in the marine strain UCYN-A (a general trend is that
marine species have smaller chromosomes) to about 9.05 Mb in the faculta-
tive symbiont
Nostoc punctiforme
ATCC29133 (
Hess, 2011
;
Wang et al., 2011
).
In addition, many cyanobacteria display a small number of plasmids (a few
kilobase to several hundreds of kilobase in size). For instance,
Synechocystis
PCC 6803 possesses seven plasmids, ranging from 2.3 kb (
Chauvat, de
Vries, Van der Ende, & Van Arkel, 1986
) to 119 kb (see CyanoBase;
http://
genome.kazusa.or.jp/cyanobase/
)
. By contrast, the marine cyanobacteria
Prochlorococcus
and
Synechococcus
have no plasmids (
Hess, 2011
). This find-
ing confirms that marine species have smaller genomes (smaller chromo-
somes and no plasmids). Very interestingly, the unicellular cyanobacterium
Cyanothece
ATCC51142, which performs photosynthesis during the day
and nitrogen fixation at night, harbours a complex 5,460,377-bp genome
(
Welsh et al., 2008
). It comprises a circular chromosome (4,934,271 bp),
a smaller linear chromosome (429,701 bp), and four plasmids (ranging
from 10,244 to 39,620 bp). The 429,701-bp linear chromosome is the
only linear genetic element reported in any photosynthetic bacterium so
far. It contains a much higher percentage of genes with no assigned func-
tion (71.7% vs. 45.7%) than the large circular chromosome. Nevertheless,
it is reasonable to assume that the linear chromosome might be impor-
tant for carbon metabolism (fermentation among other processes) as it
possesses genes presumably involved in glucose and pyruvate metabolisms
(
Welsh et al., 2008
). Similarly, the 8,361,599-bp genome of the chloro-
phyll d-possessing marine cyanobacterium
Acaryochloris marina
MBIC1101
is also complex. More than 25% of its 8462 genes are distributed over nine
plasmids (ranging from 2.13 to 374 kb in size), some of which harbour-
ing presumptive DNA repair genes
recA
,
umuC
and
umuD
(
Swingley et al.,
2008
). Though cyanobacterial plasmids propagate possibly important genes
(Cyanobase), their role in the biology and physiology of cyanobacteria
remain cryptic, so far.
