CIRCADIAN GATING OF CELL-DIVISION IN CYANOBACTERIA GROWING WITH AVERAGE DOUBLING TIMES OF LESS-THAN 24 HOURS

To ascertain whether the circadian oscillator in the prokaryotic cyano bacterium Synechococcus PCC 7942 regulates the timing of cell division in rapidly growing cultures, we measured the rate of cell division, D NA content, cell size, and gene expression (monitored by luminescence of the PpsbAI::luxAB reporter) in cultures that were continuously dilu ted to maintain an approximately equal cell density. We found that pop ulations dividing at rates as rapid as once per 10 h manifest circadia n gating of cell division, since phases in which cell division slows o r stops recur with a circadian periodicity. The data clearly show that Synechococcus cells growing with doubling times that are considerably faster than once per 24 h nonetheless express robust circadian rhythm s of cell division and gene expression. Apparently Synechococcus cells are able to simultaneously sustain two timing circuits that express s ignificantly different periods.