A small and compact genome in the marine cyanobacterium Prochlorococcus marinus CCMP 1375: lack of an intron in the gene for tRNA(Leu)(UAA) and a single copy of the rRNA operon.

Data obtained by pulsed field gel electrophoresis revealed for Prochlorococcus marinus CCMP 1375 a genome size of 1.81+/-0.04 Mbp. This value is significantly smaller than for all other cyanobacteria investigated so far. The absence of an intron in the gene for tRNA(Leu)(UAA), which otherwise is widespread among cyanobacteria, and the additional finding that the ribosomal operon exists as a single copy suggest that the deletion of non-essential sequences played a major role in the evolution of P. marinus. A small genome may have been advantageous in the adaptation to very oligotrophic marine conditions.