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After treatment with N-methyl-N'-nitro-N-nitrosoguanidine, 133 independent mutants of a haploid strain of Escherichia coli able to use phenyl-beta-galactoside as a carbon source were obtained. The galactoside was specific in selecting for mutants with increases in their uninduced levels of beta-galactosidase. Virtually all mutants (37 in a subsample of 38) carried mutations in the lac repressor gene. There were two classes of repressor mutants. As well as the commonly identified class of mutants with completely inactivated repressors, there was a frequent class of mutants (21/37) whose repressors were partially inactivated. Most of these (15/21) repressed beta-galactosidase synthesis 4 to 50 times less than wild type, but were more numerous in the lower part of this range. Their beta-galactosidase was inducible to levels characteristic of the parent strain. The repressor activities were diverse and stably expressed under routine growth conditions. The decreased activity did not result from the formation of temperature-sensitive repressors. None of the mutants with completely inactivated repressors appeared to carry UAG or UGA chain-terminating codons. On the assumption that the partially defective repressor mutants carried missense mutations, it is argued that missense mutations in the lac repressor gene modify the repressor's affinity for the operator with high probability. An explanation is proposed for the apparent sensitivity of this repressor function to partial inactivation as the result of amino acid substitutions.

Mutations Partially Inactivating the Lactose Repressor of Escherichia coli