Ycf12 is a new reaction center component in Chlamydomonas photosystem II

Natsuko Inoue-Kashino and Yuichiro Takahashi

Graduate School of Natural Science and Technology, Okayama University, Japan

One of the hypothetical chloroplast open reading frame (ycf), ycf12, is conserved in the genomes of cyanobacteria and chloroplasts except angiosperm. Recently, it was reported that the gene product, Ycf12, is a new photosystem II (PS II) component in a cyanobacterium, Thermosynechococcus elongatus (1). We investigated the expression of ycf12 in hlamydomonas reinhardtii and found the presence of Ycf12 in the thylakoid by immunoblotting using specific antibody raised against a synthetic oligopeptide of Ycf12. By fractionating thylakoid extracts with sucrose density gradient ultracentrifugation and ion exchange chromatography, Ycf12 was co-purified with PSII core complex, confirming that Ycf12 is a core subunit also in C. reinhardtii. Partial disintegration of PS II core complexes by use of chaotropic ion, KSCN, into reaction center (RC), CP47 and CP43 sub-complexes revealed that Ycf12 is co-purified with RC complex (D1/D2 heterodimer). This is inconsistent with the speculation from the crystal structure of PS II core complex from T. elongatus that Ycf12 might be associated with CP43 (1). To clarify the function of Ycf12 in the PS II complexes, we have generated ycf12-deficient transformants (Δycf12) by chloroplast transformation. It was revealed that, in ΔYcf12 thylakoid, major PS II subunits accumulate at wild-type level except that Ycf12 is totally absent. ΔYcf12 cells grew photosynthetically under moderate light condition at almost the same rate as wild-type cells. Under high irradiance, the growth of ΔYcf12 was slightly suppressed compared to the wild-type cells, which suggests that Ycf12 is not essential but plays a role to support optimal function of PS II under stress environments. 1. Kashino et al. Biochim. Biophys. Acta 1767, 1269Ð1275 (2007)

e-mail address of presenting author: nazco@land.linkclub.or.jp