Control of Ribosome Synthesis During the Cell Division Cycles of E. coli and Synechococcus

The regulation of ribosome synthesis has been investigated for nearly five decades. In earlier studies, the control of rRNA synthesis in bacteria was found to be dependent on nutrient composition of the growth media or cell growth rates, and these observations led to the growth rate-dependent regulation model. Also developed were stringent control, feedback ribosome synthesis, passive regulation, and antitermination models. Current evidence indicates that upstream (UP) element, molecular effectors, ppGpp and iNTP (initiating nucleoside triphosphate), and trans-acting proteins, Fis and H-NS, play important roles in the control of rRNA synthesis in response to changing nutritional environments. The mechanisms for the ribosome feedback regulation, and growth rate-dependent controls of rRNA synthesis remain to be determined despite numerous investigations. r-protein synthesis can be controlled by translational coupling, translation repression, or premature transcription termination. In Synechococcus, a photoautotroph, ribosome synthesis occurs early in the cell cycle as programmed events under conditions that support balanced growth. Periods of r-protein synthesis occur before rRNA synthesis periods, and rRNA synthesis is stimulated by a light-activated gene regulatory protein. These observations suggest that gene regulatory proteins are involved in the coordinate regulation of ribosome assembly in Synechococcus.