Role of rel gene in translation during amino acid starvation in Escherichia coli

Abstract

IT has been thought that the rel gene in Escherichia coli is responsible for the stringent control of stable RNA synthesis dependent on a supply of amino acids, since the gene was identified as the site of mutation to the relaxed phenotype¹. But in our previous reports^2,3, we suggested that mutational defects in rel⁻ cells are probably in the translational mechinery, the integrity of which may be required for stringent control. This supposition is based on the following observations: (1) When rel⁻ cells are shifted down in carbon or nitrogen source, the stringent control of RNA synthesis is observed in spite of their rel⁻ genotype^2,4. Only when deprived of amino acids do rel⁻ cells fail to restrict RNA synthesis, in contrast to cells having the rel⁺ allele. (2) Ribosomal inhibitors like chloramphenicol abolish the stringent control in both rel⁺ and rel⁻ cells^2,5,6. Furthermore, a cold-sensitive mutant defective in protein synthesis at low temperature earring a mutation in spcA locus, also shows the relaxed phenotype under non-permissive conditions³. It has been reported that several other mutants of rel⁺ strains having temperature-sensitive phenylalanyl-tRNA synthetase⁷, peptidyl-tRNA hydrolase⁸, or elongation factor G (ref. 9) all failed to control the synthesis of RNA at the higher temperature. This suggests that normal synthesis of protein is required for stringent control. (3) During shift-down of a carbon or nitrogen source there is no difference between the growth of rel⁺ and rel⁻ cells. Only after shift-down of amino acids do rel⁻ cells require a much longer lag period before they resume growth^2,10. This is due to the inability of rel⁻ cells to carry out the normal synthesis of protein when the supply of amino-acids is limited.