Wolfgang R. Hess

The cell cycle of the chlorophyll b -possessing marine cyanobacterium Prochlorococcus is highly synchronized under natural conditions. To understand the underlying molecular mechanisms we cloned and sequenced dnaA and ftsZ , two key cell cycle-associated genes, and studied their expression. An axenic culture of Prochlorococcus sp. strain PCC 9511 was grown in a turbidostat with a 12 h–12 h light-dark cycle for 2 weeks. During the light periods, a dynamic light regimen was used in order to simulate the natural conditions found in the upper layers of the world's oceans. This treatment resulted in strong cell cycle synchronization that was monitored by flow cytometry. The steady-state mRNA levels of dnaA and ftsZ were monitored at 4-h intervals during four consecutive division cycles. Both genes exhibited clear diel expression patterns with mRNA maxima during the replication (S) phase. Western blot experiments indicated that the peak of FtsZ concentration occurred at night, i.e., at the time of cell division. Thus, the transcript accumulation of genes involved in replication and division is coordinated in Prochlorococcus sp. strain PCC 9511 and might be crucial for determining the timing of DNA replication and cell division.

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.

An open reading frame encoding a polypeptide with significant homology (about 28% identity and 16% similarity) to human aspartoacylase (ASPA) was identified in the genome of Prochlorococcus marinus CCMP 1375, an oxyphototrophic bacterium. Sequence alignments show that, in particular, the regions previously suggested to form the catalytic core of human ASPA are evolutionarily conserved and nearly identical to that found in the Prochlorococcus putative ASPA. A glutamate at position 273 is located central to the strongly conserved INEAAY motif and corresponds to a glutamate-285 in human ASPA. A point mutation at this site, resulting in a Glu285Ala missense mutation, was identified previously as the molecular basis for Canavan disease, a form of leukodystrophy. The data provide new insights about conserved and hence functionally important domains of ASPA. The detection of this gene in an unicellular prokaryote demonstrates that this enzymatic activity is more widespread than expected previously. Furthermore, bacteria may be used as simple model organisms to study different forms of ASPA.