GENOMES - Cyanobacteria - page 110

sp. strain CC9311 where as there are 8 such genes in Synechococcus sp. strain WH8102. This suggests
that Synechococcus sp. strain CC9311 is adapted to grow in HL habitats (Rocap et al. , 2003). In
S. elongatus PCC 6301 there are 139 genes coding for photosynthesis-related components (Sugita
et al ., 2006). In Prochlorococcus strains light regulation of photosynthetic CO 2 assimilation seems to
have been lost as genes ( ndh4 , ndhF4 , CphX ) regulating low-affi nity carbon transport are missing as
well as genes encoding a thioredoxin and ferredoxin. These genes are characteristically represented
in all genomes of Synechococcus sequenced to date (Dufresne et al ., 2008).
Photosynthetic pigments : Phycobilisomes (PBSs) are highly organized complexes of phycobiliproteins
(PBPs) that contain allophycocyanins (APCs) as a central core to which the phycocyanins (PCs) and
phycoerythrins (PEs) are attached to give rise to radiating arms or rods. All the PBPs in cyanobacteria
consist of two distinct subunits α and β and are assembled into trimeric (αβ) 3 or mostly as hexameric
(αβ) 6 complexes. In freshwater Synechococcus strains ( S . elongatus PCC 7942 and Synechocystis sp. strain
PCC 6803) while the core is composed of APC, the rods are entirely composed of PC. In PE-containing
strains of Synechococcus , one basal disc in the rod is composed of PC and the rest is occupied by the
red coloured phycoerythrobilin (PEB) at a molecular ratio of 1:2. In Synechococcus sp. strain WH7805
at the basal part of the rod instead of PC, an optically variant form of PC is present that is linked
to PC and PEB at a molecular ratio of 2:1. However, in most PE-containing strains the distal part of
the rods consists of two types of PE (PEI and PEII). PEII always binds with PEB and another orange
coloured pigment known as phycourobilin (PUB; A max =495 nm) whereas PEI binds only PEB or both
PEB and PUB. Six et al . (2007) recognized three major pigment types in Synechococcus where type
1 is represented by forms with PC (strains RS9917 and WH5701), type 2 have PC and PEI (strains
WH8018 and WH7805) and type 3 have PC, PEI and PEII. Type 3 has further been subdivided into
four subtypes 3a-3d depending on the ratio of PEB and PUB bound to PEs. Strains that belong to
subtype 3a have a low ratio of PEB and PUB (WH7803) whereas those included in 3b and 3c exhibit
moderate and high ratios of PEB and PUB, respectively (strains CC9605, WH8102, RCC307, BL107
and CC9902). The fourth subtype 3d is characterized by the strains that possess variable amounts of
PEB and PUB depending on the quality of light (white/green or blue) under which they are grown
(strains CC9311 and RS9916). These show type IV chromatic adaptation. They proposed models
for PBS structure for different Synechococcus pigment types and subtypes (Fig. 10). Besides the 11
strains of Synechococcus whose genomes have been sequenced (Table 3), Six et al (2007) included eight
other strains that belonged to pigment type 2 (strain WH8018), pigment type 3a (strains AlmoO3
and RS9912), pigment type 3c (strains WH8103 and Oli31) and pigment type 3d (strains WH8020,
MI I.I and M16.17strains). As in other cyanobacteria, the PBS genes are clustered together in these
19 strains of Synechococcus . The fi rst small cluster is represented by four APC core genes apcE - A - B -C
and two other APC core genes apcD and apcF are distributed away from PBS genes. Most of the
rod genes exist in large clusters whose size depends on the complexity of pigment type, i.e. from
9-10 kbp in pigment type 1 to 27-28.5 kbp in pigment subtype 3d. The distribution and number of
genes in the three pigment types is depicted in Table 8.
The assembly of PBS is mediated by linker polypeptides and each trimeric or hexameric
subassembly contains at least one specifi c liker polypeptide. This in turn determines the type, location
and aggregation state of the PBP within the rod and also modulates the spectroscopic properties.
Glazer (1985) has provided a system of abbreviations for the linkers and their characterization
for sequence and molecular weight determinations has been accomplished. Most of the linker
polypeptides are associated with APC, PC or PE and so designated as APC-associated linkers (L C , i.e.
core linker), PC and PE associated linkers are (L R for rod linker) where as L RC is designated for PBSs