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Using a simple three-step procedure, we have isolated thermosensitive mutants ofE. coli that are specifically defective in transfer RNA (tRNA) synthesis. Our procedure was designed to identify mutants that are unable to make su3 + tRNATyr or grow at the restrictive temperature, yet under the same conditions they retain the ability to make mRNA and protein. The mutants obtained have been analyzed, and they are defective in different steps in the synthesis of functional tRNA at the restrictive temperature. Some of them may fail to modify certain bases in the tRNA. Two mutants are unable to process the 5′ end of tRNA precursor molecules. Three are unable to cleave precursor molecules at the 3′ end. 11 Mutants can not synthesize any tRNA molecules or tRNA precursors. We speculate that these latter mutants may be defective in RNA polymerase or in an RNA polymerase factor specific for stable RNA synthesis.

Mutants of Escherichia coli Thermosensitive for the Synthesis of Transfer RNA