Molekulare Basis der Genkontrolle

Mutations in the araC gene of Salmonella typhimurium LT2 which affect both activator and auto-regulatory functions of the AraC protein

The araC gene encodes a regulatory protein, AraC, that acts as both an activator and a repressor of transcription of the genes involved in the transport and catabolism of L-arabinose in Salmonella typhimurium LT2. Five araC mutants which have altered regulatory properties were characterized. All are point mutations which would result in amino acid substitutions near the C terminus of AraC. Each mutation results in altered activator and auto-regulatory AraC function in vivo. In vitro DNA-binding assays showed that three mutant AraC have measurable lowered affinity for ara controlling site DNA. The data are consistent with a model in which there is a DNA-binding domain in the C terminus of AraC which functions in both activation and repression.

Studies on the domain structure of the Salmonella typhimurium AraC protein

The Salmonella typhimurium araC gene product is known to be susceptible to proteolytic degradation. Limited cleavage by trypsin, kallikrein, elastase and pronase E yields stable fragments comprising approximately the N-terminal two thirds of the AraC protein. These fragments have in common the ability to dimerize in solution and to bind L-arabinose and D-fucose. Under appropriate conditions, hydrolysis of the AraC protein with Staphylococcus aureus V8 protease leads to a small C-terminal fragment which is able to bind specifically to a synthetic ara consensus sequence. These results indicate that, as with several other prokaryotic gene regulatory proteins, the basic functions of effector binding, subunit interaction and specific DNA binding are segregated into distinct domains of the AraC protein.

Mutations in the araC regulatory gene of Escherichia coli B/r that affect repressor and activator functions of AraC protein

Mutations in the araC gene of Escherichia coli B/r were isolated which alter both activation of the araBAD operon expression and autoregulation. The mutations were isolated on an araC-containing plasmid by hydroxylamine mutagenesis of plasmid DNA. The mutant phenotype selected was the inability to autoregulate. The DNA sequence of 16 mutants was determined and found to consist of seven different missense mutations located within the distal third of the araC gene. Enzyme activities revealed that each araC mutation had altered both autoregulatory and activator functions of AraC protein. The mutational analysis presented in this paper suggests that both autoregulatory and activator functions are localized to the same determinants of the AraC protein and that the amino acid sequence within the carboxy-terminal region of AraC protein is important for site-specific DNA binding.