Controlling cell cycle and cell fate: common strategies in prokaryotes and eukaryotes.

Generating cells of different fate within a group of equipotent cells is the underlying principle of the development of complex multicellular organisms. Cell–cell signaling or environmental differences may cause two equipotent daughter cells to assume different fates. Alternatively, asymmetric distribution of cell fate determinants during cell division can generate two cells with different fates. In either case, developmental cues have to be interpreted correctly by the machinery that controls cell cycle progression and by the programs that induce cell specification. Despite great advances in identifying the proteins that govern cell cycle progression, little is known about how their activity is modulated by developmental signals. In this issue of the Proceedings , Quon et al. (1) provide new insights into understanding this fundamental problem. They identified an inhibitor of DNA replication whose activity is temporally and spatially regulated, thereby restricting its activity during development. However, these findings were not made, as one might have expected, in a eukaryotic organism, but in the eubacterium Caulobacter crescentus . The similarities in the regulation of this bacterial cell cycle regulator and cell fate determinant and that of eukaryotic cell cycle regulators and cell fate determinants will be discussed in this commentary. Cell division in Caulobacter is asymmetric (2). A sessile stalked cell divides to give rise to two daughter cells, a stalked cell and a motile swarmer cell (Fig. 1 A ). The stalked cell is capable of immediately replicating its chromosome and subsequently dividing, whereas the swarmer cell must differentiate into a stalked cell before it can initiate DNA replication and cell division. The generation of a stalked cell and a swarmer cell during cell division and the initiation of DNA replication after the transition of a swarmer cell into a stalked cell are ultimately controlled by the CtrA protein (for cell cycle …