Paucity of sites mutable to constitutivity in the araC activator gene of the L-arabinose operon of Escherichia coli.

A large number of high-level and low-level constitutive mutations in the or&? gene of E~cherichkz COG were shown by deletion mapping to lie almost exclusively in two regions of the araC gene. Recombination data show that the high-level constitutive mutations are located within two very small regions, each probably less than ten base-pairs, while the low-level constitutive mutations are spread over two broader areas, each centered at the same two regions. All constitutive mutations isolated in either the presence or absence of D-fUCOSC, an analog of I,-arabinose which antagonizes induction by arabinose, are altered from the wild type in their response to this analog. A nonsense mutation that maps in one of the constitutive regions can be suppressed to wild type, “low-level” constitutive, or “high-level” constitutive phenotypes, depending on the amino acid inserted at the site of the mutation. This demonstrates that changing a single amino acid can cause dramatic alterations in the regulatory properties of the araC activator protein.