COEXISTENCE OF PHYCOERYTHRIN AND A CHLOROPHYLL A B ANTENNA IN A MARINE PROKARYOTE/

Prochlorococcus marinus CCMP 1375, a ubiquitous and ecologically impor tant marine prochlorophyte, was found to possess functional genes codi ng for the alpha and beta subunits of a phycobiliprotein, The latter i s similar to phycoerythrins (PE) from marine Synechococcus cyanobacter ia and bind a phycourobilin-like pigment as the major chromophore, How ever, differences in the sequences of the alpha and beta chains compar ed with known PE subunits and the presence of a single bilin attachmen t site on the alpha subunit designate it as a novel PE type, which we propose naming PE-III, P. marinas is the sole prokaryotic organism kno wn so far that contains chlorophylls a and b as well as phycobilins. T hese data strongly suggest that the common ancestor of prochlorophytes and the Synechococcus cyanobacteria contained phycobilins, Flow cytom etric data from the tropical Pacific Ocean provide evidence that deep populations of Prochlorococcus possess low amounts of a PE-like pigmen t, which could serve either in light harvesting or nitrogen storage or both.