IMMUNOLOGICAL AND ULTRASTRUCTURAL CHARACTERIZATION OF THE PHOTOSYNTHETIC COMPLEXES OF THE PROCHLOROPHYTE PROCHLOROCOCCUS (OXYCHLOROBACTERIA)

Ultrastructural features and immunological properties of some thylakoi d proteins were examined in two strains of the prochlorophyte Prochlor ococcus and compared to those of other photosynthetic prokaryotes and eukaryotes. Both strains exhibited two or three rows of tightly appres sed thylakoidal membranes, located at the cell periphery. However, thy lakoids were concentrically arranged in the strain from the Sargasso S ea (SARG) and horseshoe-shaped in the Mediterranean isolate (CCMP 1378 ). Although backing phycobilisomes, both cell types shared with cyanob acteria the presence of carboxysome-like structures and glycogen granu les as storage compounds. The main thylakoid polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were charac terized by Western blotting using several antibodies. The 30-kDa polyp eptide of the light-harvesting complex (LHC) of Prochlorococcus showed a weak positive immunological cross-reaction with an antibody raised against the 32-kDa apoprotein of the LHC of the prochlorophyte Prochlo rothrix hollandica. In contrast, it showed no immunological relationsh ips with the chlorophyll a/b (Chl a/b) LHCs of green algae and higher plants. Protein membranes from Prochlorococcus strongly cross-reacted with antibodies raised against reaction center polypeptides of photosy stems II and I (PSs II and I) of other photosynthetic organisms, confi rming the high degree of conservation of these basic compounds of the photosynthetic machinery during evolution. Immunolocalization of thyla koid proteins showed that the LHC proteins, the major PS II reaction c enter proteins (CP 43 and D2), and the PS I reaction center proteins w ere equally distributed within the thylakoid membranes in contrast to the segregation observed in higher plants and green alga thylakoids. W e also identified ribulose-1,5-bisphosphate carboxybase/oxygenase in t he carboxysomes. These results suggest that Prochlorococcus is more cl osely related to cyanobacteria than to green plastids even though in c ontains Chl b.