Received December 26, 2005
Returned for revision January 30, 2006
Accepted April 4, 2006
The Significance of C16-Fatty Acids in the sn-2 Positions of Glycerolipids in the Photosynthetic Growth of Synechocystis sp. PCC6803
Kumiko Okazaki , Norihiro Sato , Noriko Tsuji , Mikio Tsuzuki , and Ikuo Nishida *
Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyou-ku, Tokyo, 113-0033, Japan
School of Life Science, Tokyo University of Pharmacy and Life Science, Horino-uchi 1432-1, Hachioji, Tokyo, 192-0392, Japan
Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama University, Shimo-Okubo 255, Sakura-Ku, Saitama-Shi, Saitama, 338-8570, Japan
* Corresponding author; email: nishida{at}molbiol.saitama-u.ac.jp.
Most extant cyanobacteria contain C16-fatty acids in the sn-2 positions of glycerolipids, which are regulated by lysophosphatidic acid acyltransferase (LPAAT, EC 2.3.1.51). Synechocystis sp. PCC6803 contains sll1848, sll1752, and slr2060 as putative acyltransferase genes. sll1848 was recently reported to encode an indispensable palmitoyl-specific LPAAT; however, here, we show that each of the three genes is dispensable. 
1848 and 1848 2060 cells had markedly higher contents of stearate, oleate, and linoleate in place of palmitate in the sn-2 positions, suggesting that 1848 2060 cells incorporate stearate and oleate in the sn-2 positions. The levels of sll1752 transcripts increased in 1848 2060 cells. This was accompanied by increased LPAAT activity toward 18:0-CoA in the membrane fraction. From these findings, together with the activity of a recombinant sll1752 protein and complementation of the Escherichia coli plsC mutant, we conclude that sll1752 encodes a second LPAAT that prefers stearoyl and oleoyl substrates. D1848 2060 cells grew slowly at 30 °C at lower cell density, and exhibited more severe damage at 20 °C than wild-type cells. Furthermore, 1848 2060 cells exhibited photoinhibition more severely than wild-type cells. A phycobilisome core-membrane linker protein (slr0335) was also found to be susceptible to protein extraction under our conditions; its content decreased in the membrane fractions of 1848 2060 cells. We conclude that C16-fatty acids in sn-2 positions are preferred in the photosynthetic growth of this cyanobacterium, despite sll1752 orthologs being conserved in most cyanobacteria. However, no sll1752 ortholog is conserved among photosynthetic eukaryotes including Cyanidioschyzon merolae.
