Specific loss of LHCII phosphorylation in the Lemna mutant 1073 lacking the cytochrome b6/ f complex

Abstract

The thylakoid protein kinase(s) activity of Lemna perpusilla strain 6746 (wild type, WT) and the cytochrome (cyt) b 6/ f-less mutant 1073 was compared. Isolated thylakoids of both WT and mutant phosphorylated the polypeptides of 9–15, 29, 32–34 and 40–45 kDa. This kinase(s) activity was light-dependent and could be elicited by addition of duroquinol in the dark. Thylakoids from both WT and mutant phosphorylated histone III-S at comparable rates. However, the redox-controlled phosphorylation of the LHCII polypeptide which could be demonstrated in vitro and in vivo in the WT thylakoids could not be detected under any experimental condition in the cyt b 6/ f-less thylakoids. Halogenated quinone analogues known to inhibit reduction of the cyt b 6/ f complex inhibited both the electron flow and duroquinol-activated LHCII phosphorylation, but had no effect on the duroquinol-dependent phosphorylation of the other thylakoid polypeptides. These results indicate that the Lemna thylakoids contain at least two redox-activated protein kinase(s). A quinone-binding site is involved in the activation of the LHCII kinase system which is rendered inactive in the absence of the cyt b 6/ f complex.