GENOMES - Cyanobacteria - page 107

desaturase gene is present in three copies. In other nine strains of Synechococcus sp., Δ 9 and Δ 12 type
of desaturases are seen with a variation in the total number of genes from two to four. Likewise,
in all Prochlorococcus strains Δ 9 and Δ 12 type of desaturases have been encountered with a variation
in the total number of genes from two to four. Exceptionally, in G . violaceus PCC 7421, Δ 9 and Δ 12
types of desaturases are present with six genes of the former and two genes of the latter type. In
heterocystous nitrogen-fi xing forms ( Anabaena sp. strain PCC 7120, A . variabilis ATCC 29413, N .
punctiforme ATCC 29133) three types desaturases Δ 9 , Δ 12 and Δ 15 have been detected with total number
of genes being four to fi ve. However, in No. spumigena CCY9914 besides Δ 9 , Δ 12 , Δ 15 desaturases, an
additional Δ 6 type of desaturase has also been met with. Likewise, Cyanothece sp. CCY0110, Lyngbya sp.
PCC 8106 and Synechocystis sp. strain PCC 6803 also possess four types of fatty acid desaturases.
5) Photosynthesis and respiration : Genes related to the synthesis of proteins of PSI are designated
as Psa genes while those associated with PSII are named as Psb genes. The PSI is composed of 11-14
subunits designated as PsaA to PsaO. There is a great degree of similarity between the subunits of
green plants, eukaryotic algae and cyanobacteria except that in cyanobacteria genes encoding PsaG
and PsaH are absent (Xu et al ., 2001; Scheller et al ., 2001). Furthermore, cyanobacterial PSI does not
contain the cluster of light-harvesting proteins (LHCI) characteristically found in the PSI of eukaryotic
algae and higher plants. The cyanobacterial PSI complex exists in a trimeric form (Jordan et al ., 2001)
whereas the PSI of higher plants, green algae and red algae exists in a monomeric state (Gardian et
al ., 2007). Though the electron transport chain of PSI in eukaryotic algae and cyanobacteria is very
similar, there exist certain differences in the number of chlorophyll molecules associated with PSI
and PSII. The crystal structure of the different components of photosynthetic electron transport chain
have been published that relate to PSI (Jordan et al ., 2001), cytochrome b6/f (Stroebel et al ., 2003),
LHCI-PSI (Amunts et al ., 2007) and PSII (Umena et al ., 2011). Allen et al . (2011) integrated these and
proposed a molecular model of the Z-scheme of photosynthesis. This gives an idea of the various
component proteins of PSI and PSII and their arrangement in the thylakoid (Fig. 9) which makes it
easier to understand the genes related to PSI and PSII of cyanobacteria presented below.
Genes allocated for photosynthesis and respiration are found to be 124 and 141 in case of
T . elongatus BP-1 and Synechocystis sp. strain PCC 6803, respectively. In the other two cyanobacterial
genomes, i.e. G . violaceus PCC 7421 (158 genes) and Anabaena sp. strain PCC 7120 (154 genes), almost
same number of genes is found (Table 5). Kaneko et al . (1996) identifi ed 124 genes in Synechocystis sp.
strain PCC 6803 relating to PSI and PSII, phycobilisome formation, ATP synthesis, CO 2 fi xation and
electron transport systems. In the genome of T . elongatus BP-1, the gene psbV that encodes cytochrome
C550 existed in a tandemly duplicated state. There is one copy of psbA that encodes reaction centre
D1 complex of PSII and one copy of each gene of the cytochrome b6/f complex ( petA - petD , petG , petM
and petN ). Complete sets of genes for phycobilisome components ( cpcA - cpcG for phycocyanin and
apcA - apcF for allophycocyanin) are present in this organism but the absence of genes petL (encoding
a membrane spanning component) and petE (that encodes plastocyanin) is signifi cant (Nakamura et
al ., 2002). In the genome of N . punctiforme PCC 73102 genes psbA (four complete and three truncated
copies), psbD (one complete and one truncated copy) and single copies of psaA and psaB encoding
proteins of photosynthetic electron transport are present in a putative operon. Genes of the cytochrome
b6/f complex petB (cytochromeB)- petD (subunit IV)- petC (Fe/S protein)- petA (Cytochrome f)-
petE (plastocyanin) and petJ (cytochromec553) are present with an additional copy of the gene petJ .
The synthesis of plastocyanin in presence of suffi cient amounts of copper and cytochrome c553 in
presence of iron are indicated as soluble electron carriers between the cytochrome b6/f and PSI
complexes. Ferredoxin-NADP oxidoreductase and ferredoxin are encoded by a single gene ( petH )