# 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
A working model has been constructed [111] which indicates that timely appearances of macromolecular synthesis are controlled by timely coordinated light-activated gene regulatory proteins. The production of ribosomes in _Synechococcus_ is one of the major cell events and displays unique features that differ in some ways from ribosome production in _E. coli_. The synthesis of r-protein and rRNA, and the assembly of ribosomes occur early in the cell division cycle as regularly programmed events. Two _rm_ operons were identified in _Synechococcus_, rather than the seven _rm_ operons found in _E. coli_. _In vitro_ experiments [111] indicate the synthesis of three rRNA transcripts that differ in molecular size, suggesting the existence of three promoters in the _rm_A operon. Base sequence analyses have, indeed, located three promoters in _rm_A [111]. A G\({}_{6}\) tract was found upstream of _rm_ A P1, although the function of the G\({}_{6}\) tract in stimulating rRNA transcription is not known. In _E. coli_ A-tracks upstream of the UP element of _rm_ P-INTPure found to strengthen the promoter activity in stimulating rRNA transcription [111]. There were, however, sites found in _rm_ operons of _E.coli_ that are similar to those present in _Synechococcus_. A base sequence resembling the UP element were found upstream of _rm_ A P1, and sequences resembling the antitermination sequences found in _E.coli_ had been found in the leader region. In addition, sequences similar to box B of _E.coli_ were identified within the first stem and loop structure in the leader region of _Synechococcus_[111]. The rRNA synthesis period is about three hours in synchronized cultures growing at a doubling time of 8 h. Any scheme in ribosome production in _Synechococcus_ must be consistent with the following observations. (1) rRNA synthesis occurs early in the cell cycle as a genetically programmed cell event. (2) rRNA synthesis must be coordinated with r-protein synthesis such that r protein synthesis occurs before rRNA synthesis, and not in the reverse order. (3) rRNA transcription is stimulated by a light-responsive gene regulatory protein that activates rRNA synthesis in light but not in darkness ([PERSON], 1998). (4) Down regulation of rRNA and r-protein syntheses must also be coordinated so that ribosome synthesis does not occur in excess, or within the S and D\({}_{2}\) periods. Possible models of coordinate regulation of r-protein and rRNA synthesis were described elsewhere ([PERSON], 1983). Fig. 1 depicts a revised model in coordinate regulation of r-protein and rRNA syntheses. In this model, _rpb1_ is part of a hypothetical r-protein operon (Fig 1B). As r-protein is transcribed and translated Rpb1 is also produced. Rpb1 is activated by light and binds to the _rpb1_ binding site located downstream of _rmA_ P1 on rDNA ([PERSON], 1998). RNAP binding to _rm_ P1 promoter is enhanced by Rpb1 and the proposed UP element. The binding of both RNAP and Rpb1 transforms the closed complex into open complex and rRNA transcription proceeds. As rRNA is being transcribed, r-proteins bind to r-protein binding  
Figure 1: Coordinate Regulation of Ribosome Synthesis: A. Physical map of the promoter region of _rmA_ operon B. Rbp1 serves as a light responsive coordinating factor. r-protein operon displayed in Fig 1B is an idealized operon harboring the hypothetical rbp1 locus. As the r-protein operon is transcribed and translated r-proteins and Rbp1 are produced. Rbp1 is light-activated and binds to the Rbp1 binding site. C. 16S rRNA transcription and assembly of 30S ribosomal subunit. Ribosome assembly is modeled according to [PERSON] and [PERSON] (1979). The implication of the ribosome assembly is described in the text. Although not described in Fig 1, excess concentrations of r-proteins will repress mRNA translation, and the production of r-proteins, including Rbp1, will be curtailed. As a result, the production of r-protein and rRNA syntheses will decline.