Part 2

Course video 64 of 68

The arabinose system represents the first demonstration of a positive regulation of gene expression. The first araC mutants do not synthesize any of the catabolic enzymes encoded by the adjacent araBAD genes. They also fail to express AraE, the main arabinose permease, whose gene is located on the opposite side of the bacterial chromosome, far away from the araCBAD locus. In this paper, Engelsberg et al. isolate and characterize constitutive mutants. They first show that all araC constitutive and negative mutants map to the same gene. They demonstrate a coordinate expression of all tested arabinose enzymes, including the distantly located permease; this expression is specific since the level of an unrelated enzyme and of a glucose permease are unaffected. Using partial diploids, they show that the araC negative mutants are recessive to both the normal gene and the constitutive mutants.These results are discussed in the context of the known mutants of the lactose system. Other interpretations of the results will be discussed.The pioneering work will be placed in perspective with what is now known about the regulation of the arabinose enzyme, including the fact that AraC is also a repressor, and that repression involves DNA looping between distant sites.

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