# Control of Ribosome Synthesis During the Cell Division Cycles of _E. coli_ and _Synechococcus_
## 2 Control of r-protein synthesis in _E. coli_  
Ribosome synthesis involves a coordinated, sequential appearance of r-protein and rRNA. Some forms of direct genetic regulation must be involved in coordinating r-protein expression and rRNA synthesis ([PERSON] and [PERSON], 1974; [PERSON], 1999). However, the elucidation of genetic mechanisms that coordinate the syntheses of r-protein and rRNA presents a formidable problem ([PERSON], 1999). Additionally, the regulation of the entire r-protein operons by gene regulatory proteins remains to be elucidated. A more amenable problem is the determination of the mechanisms that regulate r-protein and rRNA concentrations. Two mechanisms have been proposed for controlling r-protein concentration, and these control mechanisms are referred to as feedback regulation ([PERSON] and [PERSON], 1996) or autogenous control ([PERSON] and [PERSON], 1994). The first represents the translational coupling that increases the rate of r-protein translation, while the second represents the repression of translation by repressor r-protein ([PERSON] and [PERSON], 1996). The translational coupling can increase r-protein translation rate when r-protein concentration is low as explained in the following way. Genes on the upstream positions on r-protein mRNAs could melt the secondary mRNA structure and thereby facilitate the translationof genes located downstream. Alternatively, ribosomes engaged in translation might be transferred directly from the upstream genes to the initiation codons of downstream genes. In the repression of translation, one of the r-proteins, produced from r-protein mRNA, acts as an inhibitor of r-protein mRNA translation. Accordingly, when excessive amounts of r-proteins are produced a specific r-protein can bind to the mRNA transcript that codes for the r-proteins and prevent mRNA translation. The binding of r-protein to mRNA is possible since, the r-protein binding sequences on rRNA are also present in r-protein mRNA. The repression of translation by r-protein could occur when r-protein binding sites on rRNA are fully occupied or when rRNA transcription rates are decreased during ribosome assembly. The regulation of r-protein operons by translational coupling and translation repression are manifested in a variety of ways ([PERSON] and [PERSON], 1986; [PERSON] and [PERSON], 1996). One example, the translational coupling of L10 operon presents an interesting albeit a complex scheme. The L10 operon codes for L10, L7/L12 r-proteins, and the \(\beta\) and \(\beta^{\prime}\) subunits of RNA polymerase (RNAP). L7 and L12 occur as acetylated and unacetylated forms, respectively. Translation of the L10 operon is regulated by the binding of L10 or by L10, L7/L12 complex to a region located at the 5' end of _rDU_ cistron ([PERSON], 1989). However, the protein copies of L10:L7/L12:\(\beta\):\(\beta^{\prime}\) are synthesized in a ratio of 1:>4:0.2:0.2 ([PERSON] and [PERSON], 1980). A mechanism for the translational coupling of L10 operon was proposed to explain the synthesis of L10 and L7/L12 in a ratio of four to one ([PERSON] et al., 1981). It was suggested that the translation of L10 opens up the region between L10 and L7/L12 which consists of 66 base pairs. This event allows other ribosomes to initiate translation at the region such that synthesis of L7/L12 r-proteins are translated about four times as fast. It should also be noted that overproduced free L10 is rapidly degraded, and this activity could contribute to the unusual stoichiometry of L10 and L7/L12 ([PERSON], 1990). Translation repression is proposed as the major mechanism that controls r-protein concentration in relation to rRNA concentration in steady state growth or growth in rapidly changing environments. An alternative mechanism for controlling r-protein synthesis is premature transcription termination.