DECREASE OF POLYPEPTIDES IN THE PS-I ANTENNA COMPLEX WITH INCREASING GROWTH IRRADIANCE IN THE RED ALGA PORPHYRIDIUM-CRUENTUM

Thylakoids isolated from cells of the red alga Porphyridium cruentum e xhibit an increased PS I activity on a chlorophyll basis with increasi ng growth irradiance, even though the stoichiometry of Photosystems I and II in such cells shows little change (Cunningham et al. (1989) Pla nt Physiol 91:1179-1187). PS I activity was 26% greater in thylakoids of cells acclimated at 280 mu mol photons . m(-2). s(-1) (VHL) than in cells acclimated at 10 mu mol photons . m(-2). s(-1) (LL), indicating a change in the light absorbance capacity of PS I. Upon isolating PS I holocomplexes from VHL cells it was found that they contained 132 +/ - 9 Chl/P700 while those obtained from LL cells had 165 +/- 4 Chl/P700 . Examination of the polypeptide composition of PS I holocomplexes on SDS-PAGE showed a notable decrease of three polypeptides (19.5, 21.0 a nd 22 kDa) in VHL-complexes relative to LL-complexes. These polypeptid es belong to a novel LHC I complex, recently discovered in red algae ( Wolfe et al. (1994a) Nature 367:566-568), that lacks Chl b and include s at least six different polypeptides. We suggest that the decrease in PS I Chl antenna size observed with increasing irradiance is attribut able to changes occurring in the LHC I-antenna complex. Evidence for a Chl-binding antenna complex associated with PS II core complexes is l acking at this point. LHC II-type polypeptides were not observed in fu nctionally active PS II preparations (Wolfe et al. (1994b) Biochimica Biophysica Acta 1188:357-366), nor did we detect polypeptides that sho wed immunocross-reactivity with LHC II specific antisera (made to Chla mydomonas and Euglena LHC II).