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Mutations in genearaB producing anl -arabinose-negative phenotype cause either an increase (hyperinducible), decrease (polar), or have no effect at all on the inducible rate of expression of thel -arabinose operon. FourteenaraB gene mutants exhibiting such effects were shown to be the result of: nonsense, frameshift, or missense mutations. All missense mutants were hyperinducible, exhibiting approximately a twofold increase in rate ofl -arabinose isomerase production. All frameshift and most nonsense mutants exhibited polar effect. One nonsense mutant was hyperinducible. Thecis -dominant polar effect of nonsense and frameshift mutants (as compared to induced wild type) were more pronounced in arabinose-utilizing merodiploids and inaraBaraCc double mutants where inducible and constitutive enzyme levels are respectively determined. On the other hand, in arabinose-utilizing merodiploids, missense mutations no longer exhibited hyperinducibility but displayed a wild-type level of operon expression. Increases in the wild type-inducible rate of expression of the operon were found when growth rate was dependent on the concentration ofl -arabinose. Cyclic 3′,5′-adenosine monophosphate also stimulated expression of the operon with the wild type in a minerall -arabinose medium. These observations are explained on the basis that the steady-state expression of thel -arabinose operonOIBAD is dependent on the concentration of (i)l -arabinose, the effector of this system, which stimulates the expression of the operon, and (ii) catabolite repressors, produced froml -arabinose, which dampen the expression of the operon. We have termed the latter phenomenon “self-catabolite” repression.

Hyperinducibility as a Result of Mutation in Structural Genes and Self-Catabolite Repression in the ara Operon