Regulatory Gene Control of Transcription of the L-Arabinose Operon in Escherichia coZi*

The E-arabinose operon is under both positive and negative control by the product of its regulatory gene, UrQc. We have measured transcription of the arabinose genes by hybridization of 3H-labeled messenger RNA to single-stranded DNA from an arabinose transducing phage in order to study the effect of the regulatory gene on transcription. In strains in which the arabinose regulatory gene is completely nonfunctional due to deletion or nonsense mutation, only lack of activator, the positive controlling element, prevents expressionof the arabinose operon. In these strains no mRNA from the arabinose operon can be detected, indicating that the positive control is exerted on the transcription process. Arabinoseconstitutive mutants which make functional activator in the absence of arabinose or which do not need the activator function for enzyme synthesis produce mRNA from the arabinose operon even in the absence of inducer. When an araC” F’ara episome is introduced into these constitutive strains, making them diploid for the arabinose genes, the araC+ gene provides repressor function and transcription of the operon is shut off in the absence of inducer, demonstrating that negative control also functions at the transcription step. A number of araC mutants producing lower than wild type levels of enzymes from the arabinose operon were shown to have correspondingly lower levels of mRNA from the operon. These results are consistent with the hypothesis that the araC gene affects only the process of transcription of the arabinose operon.