# 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 _E.coli_ during steady state growth  
Investigations into the control of rRNA synthesis in bacteria have been ongoing for about 46 years. Described below are some of the major developments in the control of rRNA synthesis that indicate the enormously difficult problem of trying to determine the mechanisms involved in ribosome synthesis. The arduous quest in determining the fundamental mechanisms in the control of rRNA synthesis began with the publication of a classic paper on RNA content of _Salmonella typhimurium_ during balanced growth conditions ([PERSON] et al., 1958). A major finding was that RNA contents increased exponentially when plotted against linear increase of growth rates. The data indicated that the composition of the growth media controls growth rates, and growth rates, in turn, control ribosome synthesis ([PERSON] and [PERSON], 1966). Subsequent experiments indicated that the number of ribosomes per unit cellular protein is equal to the growth rate (\(\mu\)) and the rate of ribosome accumulation is roughly proportional to \(\mu^{2}\) ([PERSON], 1980). These observations gave rise to a growth rate-dependent regulation model of ribosome synthesis during steady state growth and served to guide a great number of studies on this subject. In line with the principle of growth rate-dependent regulation is ribosome feedback model which was developed to explain the control of rRNA synthesis during balanced or steady state growth ([PERSON] et al., 1983). Experiments to test this model were based on rRNA activities in relation to gene dosage of rRNA operons. The results of gene dosage experiments indicated that rRNA expression was regulated by the amount of functional rRNAs assembled in ribosomes, and the appropriate concentration of ribosomes was controlled by some form of feedback mechanisms. Originally, it was thought that non-translating ribosomes acted as feedback regulators. Upon further experiments ([PERSON] et al., 1987), translating ribosomes were proposed to be the feedback regulators. Unfortunately, the mechanism of feedback regulation by translating ribosome turned out to be very elusive and signal molecules of ribosome feedback regulation remained to be identified. Nevertheless, the notion that feedback mechanism plays a role in ribosome synthesis was found to be intriguing. That is, the expression of rRNA is controlled by feedback mechanisms and the feedback loops balanced rRNA synthesis rates with the demands for protein syntheses ([PERSON] et al., 1983; [PERSON] et al., 1987). A number of experiments were done to determine the relationship between feedback mechanism and control of rRNA transcription. In these studies, rRNA promoter activities were monitored where gene dosages of rRNA operons were increased or decreased ([PERSON] et al., 1983; [PERSON] et al., 1987; [PERSON] et al., 1993; [PERSON] et al., 2000), or in conditional expression of rRNA genes ([PERSON] et al., 1985), or when mutant forms of _fis_, ([PERSON] et al., 1990), _rpoA_ ([PERSON] et al., 1993), and _nu8_B ([PERSON] et al., 1985) were introduced into the cells. The results of these experiments showed that rRNA core promoter activities were decreased. When the corresponding changes in the rRNA core promoter activities were made rRNA synthesis rates were restored indicating that the feedback mechanisms were responsible in restoring rRNA synthesis rates in response to the demand for protein synthesis. It is now known, however, that ppGpp and iNTP serve as the feedback regulators in controlling rRNA expression, and these molecular effectors play major roles in controlling rRNA synthesis during rapidly changing environments ([PERSON] and [PERSON], 2003). The participation of ppGpp and iNTP in rRNA synthesis will be described in the next section dealing with the control of rRNA in rapidly changing environments. Nevertheless, ppGpp was once considered to be involved in the growth-rate dependent regulation of rRNA synthesis ( see [PERSON] et al., 1995).