Stringent control and protein synthesis in bacteria.

Most bacteria have evolved a number of regulatory mechanisms which allow them to maintain a balanced and rather constant cellular composition in response to nutritional variations. In particular, when the availability of any aminoacyl-tRNA species becomes limiting (namely through amino acid starvation or inactivation of an aminoacyl-tRNA synthetase), several biochemically distinct physiological processes are significantly modified. This coordinate adjustment of cellular activity is termed the "stringent response". Under such conditions of aminoacyl-tRNA limitation, protein synthesis still proceeds, but various quantitative as well as qualitative changes in polypeptide metabolism can be observed. In this review, after a brief recall of the main characteristics of the stringent response, several aspects concerning protein synthesis in deprived bacteria have been presented. First, the rates of residual protein formation, peptide chain growth and protein degradation, and the molecular weight distribution of proteins newly synthesized have been analyzed. Then, the data relative to the biosynthetic regulation of non-ribosomal and ribosomal proteins have been summarized and compared to the results obtained from in vitro experiments using transcription-translation coupled systems. Finally, the problem of translational fidelity during deprivation has been discussed in connection with the metabolic behavior of polysomal structures which are still maintained in cells. The stringent dependence of cellular activity on aminoacyl-tRNA supply is known to be abolished by single-site mutations which confer to bacteria a phenotype referred to as "relaxed". These mutant strains provide an useful analytical tool in the scope of understanding the stringency phenomenon. Therefore, their proteosynthetic activity under aminoacyl-tRNA deprivation has also been studied here, in comparison to that of normal wild-type strains.