Protein and Nucleic Acid Synthesis in Two Mutants of Escherichia coli with Temperature-Sensitive Aminoacyl Ribonucleic Acid Synthetases

Abstract

Two temperature-sensitive mutants of E. coli were isolated which grow almost normally at 30 C and fail to grow at 37 C. One (I-9) was derived from a strain with stringent, amino-acid control of ribonucleic-acid (RNA) synthesis; the other (IV-4) was derived from a strain with relaxed amino-acid control of RNA synthesis. When cultures of these mutants growing at 30 C were shifted to 37 C, IV-4 synthesized RNA preferentially to protein but I-9 did not. Cell-free extracts of both mutants and their parent strains were examined for their ability to catalyze adenosine-triphosphate (ATP)-dependent attachment of amino-acids to soluble RNA (sRNA). These measurements indicated that I-9 possesses a temperature-sensitive valyl sRNA synthetase, and that IV-4 possesses a temperature-sensitive phenyla-lanyl sRNA synthetase. The behavior of these mutants suggests that amino-acids permit RNA synthesis in stringent strains only after activation or attachment to sRNA, that relaxed strains can over-produce RNA without a complete array of fully functioning aminoacyl sRNA synthetases, and that these enzymes are obligatory for the biosynthesis of proteins.