Genetic deletion of proteins resembling Type IV pilins in Synechocystis sp. PCC 6803: their role in binding or transfer of newly synthesized chlorophyll.

Upon non-denaturing gel electrophoresis of Synechocystis sp. PCC 6803 thylakoid extracts, a Type IV pilin-like protein encoded by open reading frame sll1694 was found in chlorophyll-containing bands. The Synechocystis sp. PCC 6803 genome also encodes two similar open reading frames, sll1695 and slr1456. Even though transcripts of sll1694 and slr1456 could be detected, deletion of the three open reading frames in systems with normal chlorophyll content had no effect. However, Sll1694 was found to affect the rate of chlorophyll synthesis and of the assembly of chlorophyll-binding proteins. In the sll1694/sll1695 deletion mutant in a PS I-less/chlL- background, which is unable to synthesize chlorophyll in darkness, chlorophyll synthesis during the first hours of illumination after dark incubation was 30% slower than in the PS I-less/chlL- strain. Moreover, the biogenesis of chlorophyll-protein complexes with a 77K chlorophyll fluorescence emission maximum at 685 mm was delayed by several hours in this mutant whereas the rate of biogenesis of photosystem II was not significantly affected. Furthermore, results of non-denaturing gel electrophoresis indicated that a chlorophyll-binding complex formed during the early hours of chlorophyll synthesis was altered in stability and mobility upon deletion of the three open reading frames. We propose that the protein encoded by sll1694 is involved in, but is not absolutely required for, delivering chlorophyll to nascent photosystems and antennae.