The divinyl-chlorophyll a/b-protein complexes of two strains of the oxyphototrophic marine prokaryote Prochlorococcus - Characterization and response to changes in growth irradiance

Apoproteins of the antenna complexes of Prochlorococcus marinus clone SS120 (= CCMP 1375) and Prochlorococcus sp. clone MED4 (= CCMP 1378) cross-reacted with an antibody against the 30 kDa CP 5 complex of Prochlorothrix hollandica antenna. For the MED4 strain, which has a high divinyl-chlorophyll a to divinyl-chlorophyll b (DV-Chl a/b) ratio ranging from 11.4 to 15.0 (w/w), the major antenna proteins had an apparent molecular mass of 32.5 kDa. In contrast for the SS 120 strain, which has a low DV-Chl a/b ratio ranging from 1.1 to 2.2, antenna apoproteins were observed in the range 34-38 kDa. For both strains, these apoproteins decreased at high growth irradiance but more markedly in the latter. Partially purified antenna fractions had a DV-Chl a/b ratio ca. 7-fold lower for SS120 than for MED4 at 30 μmol photons m^-2 s^-1. For both strains, the 77 K fluorescence emission spectra of whole thylakoids displayed a major peak at 685 nm and a broad but very low shoulder above 700 nm. Energetic coupling of the antenna to both PS II and PSI reaction centers was demonstrated for SS120 by the strong contribution of DV-Chl b in both the 77 K excitation fluorescence spectra and the oxidized minus reduced absorption difference spectra of P700. The PS I to PS II ratio of Prochlorococcus SS120 was determined as being 0.7 4 ± 0.1 at low light.