Rapid evolutionary divergence of Photosystem I core subunits PsaA and PsaBin the marine prokaryote Prochlorococcus

The nucleotide sequences of the genes coding for the subunits of the Photos ystem I (PS I) core, PsaA and PsaB were determined for the marine prokaryot ic oxyphototrophs Prochlorococcus sp. MED4 (CCMP1378), P. marinus SS120 (CC MP1375) and Synechococcus sp. WH7803. Divergence of these sequences from th ose of both freshwater cyanobacteria and higher plants was remarkably high, given the conserved nature of PsaA and PsaB proteins. In particular, the P saA of marine prokaryotes showed several specific insertions and deletions with regard to known PsaA sequences. Even in between the two Prochlorococcu s strains, which correspond to two genetically different ecotypes with shif ted growth irradiance optima, the sequence identity was only 80.2% for PsaA and 88.9% for PsaB. Possible causes and implications of the fast evolution rates of these two PS I core subunits are discussed.