Escherichia coli strains with modified RNase II activity: Isolation and properties

Summary In a search for Escherichia coli strains defective in ribonuclease activity, a uracil requiring strain lacking RNase I activity was fed yeast RNA as the sole source of uracil and mutants that failed to utilize yeast RNA as the sole source of uracil were isolated. Among thirty colonies thus isolated and tested three were found to contain a heat labile RNase activity in cell free extracts. (Assays were performed under conditions for RNase II.) Since no strain completely lacking this RNase activity was found, and since these three strains have reduced growth rates that seem to be caused by the same mutation that altered the RNase activity, we concluded that RNase II activity is indispensible during cell growth.