NICHE ADAPTATION OF PROCHLOROCOCCUS ECOTYPES: INSIGHTS THROUGH COMPARATIVE GENOMICS

Hess, W. R.¹*, Rocap, G.², Steglich, C.¹, Post, A.², Ting, C. S.³ & Chisholm, S. W.^{2,3 1}Humboldt-University Berlin, Germany; Institute of Biology, Chausseestr. 117, D-10115 Berlin, Germany; ²Massachusetts Institute of Technology, Department of Civil and Environmental Engineering and ³Department of Biology, Massachusetts Institute of Technology, 15 Vassar Street, 48-425 MIT, Cambridge MA 02139, USA Prochlorococcus is an extremely small, chlorophyll b-containing oceanic cyanobacterium. Specific ecotypes of Prochlorococcus have adapted to different ecological conditions with regard to factors as light or the available nutrients. Such differentially adapted ecotypes are less than 3% divergent in their 16S rRNA sequences, which invites speculation as to how their specific gene content has diverged to reflect the particular niche of each strain. Complete genome sequences have been determined of two Prochlorococcus strains, MED4 and MIT9313. These two strains are representatives of high and low light-adapted ecotypes. Intriguing similarities between both genomes include their small size and compact organization (MED4: 1.7 Mbp and MIT9313: 2.3 Mbp), a gene cluster for RubisCo and carboxysomal proteins that is of obviously non-cyanobacterial origin, or genes for two different lycopene cyclases explaining how Prochlorococcus synthesizes alpha-carotene, a carotenoid that is not common to cyanobacteria. Several genes and operons which in cyanobacteria are involved in light harvesting, nitrate utilization or in the generation of circadian rhythms have been reduced to a different degree in the two compared genomes. MED4 has many more genes encoding high light inducible proteins and photolyases. In contrast, MIT9313 possesses more genes to build up more complex light harvesting structures, including a gene cluster to produce chromophorylated phycoerythrin. The latter represents an intermediate between the phycobiliproteins of non-chlorophyll b containing cyanobacteria and a degenerated phycoerythrin present in MED4. Screening of natural samples from the Red Sea suggests that a highly similar phycoerythrin form is wide-spread among high light-adapted ecotypes.