Prochlorococcus marinus
, the dominant photosynthetic organism in
the ocean, is found in two main ecological forms: high-light-adapted genotypes
in the upper part of the water column and low-light-adapted genotypes at the
bottom of the illuminated layer.
P. marinus
SS120, the complete
genome sequence reported here, is an extremely low-light-adapted form. The
genome of
P. marinus
SS120 is composed of a single circular
chromosome of 1,751,080 bp with an average G+C content of 36.4%. It contains
1,884 predicted protein-coding genes with an average size of 825 bp, a single
rRNA operon, and 40 tRNA genes. Together with the 1.66-Mbp genome of
P.
marinus
MED4, the genome of
P. marinus
SS120 is one of the two
smallest genomes of a photosynthetic organism known to date. It lacks many
genes that are involved in photosynthesis, DNA repair, solute uptake,
intermediary metabolism, motility, phototaxis, and other functions that are
conserved among other cyanobacteria. Systems of signal transduction and
environmental stress response show a particularly drastic reduction in the
number of components, even taking into account the small size of the SS120
genome. In contrast, housekeeping genes, which encode enzymes of amino acid,
nucleotide, cofactor, and cell wall biosynthesis, are all present. Because of
its remarkable compactness, the genome of
P. marinus
SS120 might
approximate the minimal gene complement of a photosynthetic organism.