Incubation of histidine requiring auxotrophs of the bacterium Salmonella typhimurium with malonaldehyde, a three-carbon dialdehyde, produced an increased number of revertants in specific strains. Mutagenesis was only observed in frameshift mutants with normal excision repair and did not occur in those base-pair substitution mutants tested. The results are consistent with the cross-linking of bacterial DNA by malonaldehyde leading to mutagenesis expressed through the error-prone repair system.

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The nature of the operator region of an operon in bacteria has been primarily defined in terms of the characteristics of its mutant forms. One type of mutant alteration of an operator, designated 00, results in the nonfunctioning of the associated structural genes, regardless of the conditions of growth.1 Evidence has been presented which indicates that an operator does not act via a cytoplasmic product. Jacob and Monod have suggested that it acts as a receiver of controlling signals (i.e., the receptor of the repressor) and as the initiating point for the transcription into messenger RNA of the information carried by the structural genes of Downloaded by guest on July 5, 2020

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