Photosynthesis Research 57: 183–191, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 183 Regular paper Characterization of the Photosystem I subunits PsaI and PsaL from two strains of the marine oxyphototrophic prokaryote Prochlorococcus Georg W.M. van der Staay, Seung Yeo Moon-van der Staay, Laurence Garczarek & Fr´ ed´ eric Partensky Observatoire Oc´ eanologique de Roscoff, CNRS-UPR 9042 et Universit´ e Pierre et Marie Curie, BP 74, F-29682 Roscoff Cedex, France Received 24 February 1998; accepted in revised form 7 June 1998 Key words: cyanobacteria, photosynthesis, Photosystem I, prochlorophyte, psaI, psaL Abstract A 25 kDa protein associated with Photosystem I (PS I) of the divinyl-chlorophyll a/b-containing oxychlorobac- terium Prochlorococcus marinus SS120 (CCMP 1375) was isolated, and the amino acid sequences of the N-terminus and one internal peptide were determined. Polymerase chain reaction (PCR) with degenerate primers yielded a 92 bp fragment, which was used to isolate the complete gene from a genomic library. The corresponding gene was isolated from a library of Prochlorococcus sp. MED4 (CCMP 1378). In both Prochlorococcus strains, the gene encodes a protein of 199 amino acids. The gene products show a strong sequence similarity to the PS I subunit PsaL. The N-terminus contains a hydrophilic domain that has not been found in PsaL proteins from other organisms. In both strains, sequences encoding a protein similar to PsaI were found upstream of the psaL gene. Both genes are transcribed in the same direction. Abbreviation: DV-Chl – divinyl chlorophyll Introduction Prochlorococcus is a genus of very small photo- synthetic prokaryotes very abundant in the ocean (Chisholm et al. 1992). These organisms contain DV- Chl a, DV-Chl b and a Chl c like pigment, but no monovinyl Chl a, the Chl found in all other oxypho- totrophs. Since the description of the first Prochloro- coccus species, P. marinus, several other strains have been isolated. Phylogenetic analyses showed consid- erable differences between these isolates (Scanlan et al. 1996; Urbach et al. 1998). Two strains corresponding to different Prochloro- coccus species or ecotypes have been particularly studied. P. marinus SS120 (CCMP 1375) has a DV- Chl a to b ratio of less than 1 and is adapted to low light intensities (Moore et al. 1995). This species also contains some monovinyl Chl b (Partensky et al. 1993) and at least one phycobiliprotein (Hess et al. 1996). In contrast, Prochlorococcus sp. MED4 (CCMP 1378) has a DV-Chl a to b ratio of about 10 and is adapted to high light (Moore et al. 1995). Adaptation to these light conditions appears to be genetically determined. The high light adapted strain cannot acclimate to very low light irradiances, and conversely (Moore et al. 1995). This raises questions about the mechanisms of adaptation to light in these ecologically important or- ganisms and the influence of light on the composition of their photosynthetic apparatus. Several genes encoding proteins associated with PS II have been identified so far. The DV-Chl a/b binding antenna protein is related to and probably de- rived from the IsiA-protein (La Roche et al. 1996), a protein that is induced under iron-limiting conditions in cyanobacteria (Laudenbach and Straus 1988). Par- tial sequences of the PS II protein PsbB (CP47) were shown to be similar to the corresponding ones from cyanobacteria and chloroplasts (Urbach et al. 1998),