POSTTRANSLATIONAL METHYLATION OF PHYCOBILISOMES AND OXYGEN EVOLUTION EFFICIENCY IN CYANOBACTERIA

A post-translationally methylated asparagine residue, gamma-N-methylasparagine (NMA), is found at the beta-72 site in many phycobiliproteins. We have examined the effects of asparagine methylation on photosynthetic rates in the wild-type cyanobacterium Synechococcus PCC 7942 (beta-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 exhibit a 14% decrease in fluorescence quantum yield (Swanson, R.V. and Glazer, A.N. (1990) J. Mol. Biol. 214, 787-796). Relative rates of Photosystem II electron transfer were measured for these strains by monitoring steady state rates of oxygen evolution in whole cells. The methylase-minus mutants demonstrated lower rates of electron transfer through Photosystem II under conditions in which the phycobilisome components were preferentially illuminated with orange light at low intensity. Oxygen evolution rates were indistinguishable for the wild-type and mutant strains upon selective illumination of the chlorophyll-containing light-harvesting antennae with blue light. The observed differences in photosynthetic rates are consonant with the levels of asparagine methylation in the phycobilisomes. These results support the hypothesis that asparagine methylation which is energetically costly yet evolutionarily conserved, is associated with increased energy transfer efficiency through Photosystem II.