POSTTRANSLATIONAL METHYLATION OF PHYCOBILISOMES AND OXYGEN EVOLUTION EFFICIENCY IN CYANOBACTERIA

A post-translationally methylated asparagine residue, gamma-N-methylas paragine (NMA), is found at the beta-72 site in many phycobiliproteins . We have examined the effects of asparagine methylation on photosynth etic rates in the wild-type cyanobacterium Synechococcus PCC 7942 (bet a-72 = NMA) and two Synechococcus PCC 7942 methylase mutants (beta-72 = Asn) that exhibit no detectable methylase activity. Previous studies indicate that phycobilisomes isolated from non-methylated mutants exh ibit a 14% decrease in fluorescence quantum yield (Swanson, R.V. and G lazer, A.N. (1990) J. Mol. Biol. 214, 787-796). Relative rates of Phot osystem II electron transfer were measured for these strains by monito ring steady state rates of oxygen evolution in whole cells. The methyl ase-minus mutants demonstrated lower rates of electron transfer throug h Photosystem II under conditions in which the phycobilisome component s were preferentially illuminated with orange light at low intensity. Oxygen evolution rates were indistinguishable for the wild-type and mu tant strains upon selective illumination of the chlorophyll-containing light-harvesting antennae with blue light. The observed differences i n photosynthetic rates are consonant with the levels of asparagine met hylation in the phycobilisomes. These results support the hypothesis t hat asparagine methylation which is energetically costly yet evolution arily conserved, is associated with increased energy transfer efficien cy through Photosystem II.