Eidlic, Lia (Purdue University, Lafayette, Ind.), and Frederick C. Neidhardt. Protein and nucleic acid synthesis in two mutants of Escherichia coli with temperature-sensitive aminoacyl ribonucleic acid synthetases. J. Bacteriol. 89:706–711. 1965.—Two temperature-sensitive mutants of Escherichia 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 phenylalanyl 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 overproduce RNA without a complete array of fully functioning aminoacyl sRNA synthetases, and that these enzymes are obligatory for the biosynthesis of proteins.