# Control of Ribosome Synthesis During the Cell Division Cycles of _E. coli_ and _Synechococcus_
## 2 Control of r-protein synthesis in _E. coli_
### Control of rRNA synthesis in rapidly changing environments
sites on the 5'-end of the16S rRNA as the sites become exposed during the transcription process (Fig 1C). The ribosome assembly process as depicted in Fig. 1 indicates that r-proteins are synthesized before the transcription of rRNA. The involvement of a light-activated gene regulatory protein and the coordinated transcriptions of r-protein and rRNA in the figure are consistent with the observations made in previous reports [1, 1, 13, 14]. The curtailment of r-protein and rRNA syntheses can be accomplished by negative feedback control [13]. Rpb1 synthesis is also curtailed and, as a consequence, the initiation of rRNA transcription stops as well. In support of the feedback control mechanism, a r-protein exhibiting the characteristics of a repressor r-protein has been reported in _Synechococcus_[14]. S7 is a product of _rps7_ in a r-protein operon structurally similar to _str_ operon of _E. coli_. Binding sites of S7 are found on 16S rRNA and r-protein mRNA, indicating that S7 can bind to rRNA transcripts in the formation of ribosomes or bind to mRNA of _str_ operon to hinder r-protein translation. Model 1 assumes the involvement of anti-termination and sigma factors in the regulation of rRNA synthesis in _Synechococcus_. The involvement of other ancillary molecules in stimulating rRNA transcription is not ruled out. The regulatory roles of ppGpp [12] and INTP in the control of rRNA synthesis in _Synechococcus_ have not been demonstrated. These molecular effectors do regulate promoter activities in rRNA transcriptions in _E. coli_ in conditions of amino acid starvation, or changes in nutrient composition and growth phases. It appears that these molecular effectors are not involved in coordinating r-protein and rRNA synthesis in _Synechococcus_ in conditions that support balanced growth [13]. Locating _rpb1_ on the r-protein operon as indicated in model I is not a radical proposal. Genes coding for proteins involved in transcription (\(\beta\), \(\beta_{2}\)\(\alpha\) factor), translation (elongation and initiation factors Ef-G, Ef-Tu, TufA, EF-Ts) and DNA replication (DnaG) are found in r-protein operons. **Summary, conclusions, and perspectives**  
_E. coli_ evolved multi-functional control systems of r-protein and rRNA syntheses that respond to changes in the nutritional environments. The control of r-protein synthesis at the translational level involves translational coupling and regulation of translation by repressor r-protein. r-protein transcription can be terminated prematurely by specific r-proteins as in the transcription termination of S10 operon by L4. Significant progress has been made recently in describing the participation of molecular effectors (ppGpp and INTP) and trans-acting proteins (Fis and H-NS) in controlling the transcription of rRNA synthesis in changing nutritional environments or in different growth phases. It would be of interest to know if there are other molecular effectors that could stimulate r-protein synthesis, and whether these molecular effectors could coordinate r-protein synthesis with rRNA synthesis in ribosome formation. In any case, ppGpp, INTP, Fis and H-NS proteins are not involved in the growth-rate dependent control of rRNA transcription or feedback ribosome synthesis models; regulators and regulatory mechanisms of these models remain to be determined. The transcription antitermination models of r-RNA operons involve antiterminators and r-protein molecules that permit the read-through of strategically located termination sites within _rm_ operns. There are, however, several problems that must be overcome in order to fully explain the coordinate regulation of r-protein and rRNA in ribosome synthesis. Gene regulatory proteins that stimulate r-protein operons and rm operons, and the genetic factors that coordinate the r-protein and rRNA syntheses during the progress of the cell cycle need to be determined. The resolution of these problems is will not be easy. In _Synechococcus_, sequential r-protein and rRNA synthesis periods occur early in the cell cycle during balanced growth.