Perspectives in the coordinate regulation of cell cycle events in Synechococcus.

The concepts of cell theory and the notions of coordinate regulation of the cell cycle have been known for centuries but the conundrum of coordinate regulation of the cell cycle remains to be resolved. The unique characteristics of the cell division cycle of Synechococcus, a photosynthetic bacterium, suggest the existence of a complex network of light/dark responsive gene regulatory factors that coordinate its cell cycle events. Evaluation of the highly ordered cell cycle of Synechococcus led to the construction of workable models that coordinate the cell cycle events. A central issue in bacterial cell growth is the elucidation of the genetic regulatory mechanisms that coordinate the cell cycle events. Synechococcus, a unicellular cyanobacterium, displays a peculiar cell growth cycle. In the light growth conditions, a highly ordered and sequentially coordinated appearances of r-protein synthesis, rRNA synthesis, DNA replication, chromosome segregation, and cell septum formation occur (Figs 1, 2A). Cell membrane syntheses occur predominantly during mid-cell cycle and cell division period. Synthesis of thylakoid (=photosynthetic apparatus) is thought to occur during mid-cell cycle and coincides with a period of peak phospholipid synthesis and oxygen production (Csatorday and Horvath, 1977; Asato, 1979). Cell wall syntheses occur in short discontinuous periods throughout the cell cycle and during cell division (Asato, 1984). Distinct D1 (=G1), C (S) and D2 (=G2) periods as defined by Cooper and Helmstetter (1968) are observed in synchronized cultures of Synechococcus (Asato, 1979). When light grown cultures are placed in the dark, the ongoing cell cycles are aborted in the dark (Fig. 3A) and cell divisions do not occur (Asato, 1983; Marino and Asato, 1986). Upon re-exposure of the cell cultures to the light growth conditions, about 14 h later, new cell cycles are re-initiated. These characteristics of cell growth are considered to be expressions of a unique strategy of obligate phototrophic mode of growth to perpetuate their species (Asato, 2003). Nevertheless, the intermediate metabolism, the synthesis of building block molecules, the genetics and molecular biology in the formation of major macromolecules are similarto heterotrophs such as E. coli. In any case, the genes that are involved in the formation of the cellular structures and the genes that control the orderly appearances of the cell cycle events must be coordinated by novel genetic mechanisms. Currently, there are no known physiological/physical mechanisms, growth rate dependent factors or traditional genetic regulatory mechanisms that could explain the coordinate regulation of the cell cycle events in bacteria (Newton and Ohta, 1992; Vinella and D'Ari, 1995; Donachie, 2001; Margolin and Bernander, 2004). Because the genetic mechanisms of coordinate regulation of cell cycle events in bacteria are largely unexplained, the questions on how Synechococcus coordinates the cell cycle events present a difficult problem to resolve. Nevertheless, the problems with regard to the coordinate regulation of the cell cycle events of Synechococcus must be considered. Possible solutions are developed and described in this article. The proposed schemes do not exclude the formation of other genetic mechanisms on the regulation of cell cycle events in Synechococcus. Although the cell cycle of Synechococcus is not widely known, the issues on the coordinate regulation of the cell cycle events are not trivial since similar regulatory mechanisms most likely occur in other prokaryotes.