IDENTIFICATION OF MUTATIONS IN THE ESCHERICHIA COLI RECA AND LEXA REGULATORY LOCI.
AuthorWERTMAN, KENNETH FRANKLIN.
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PublisherThe University of Arizona.
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AbstractThis report describes the development and use of an expression vector system based on the single-stranded DNA bacteriophage M13. A derivative of M13mp8, designated M13mp8/P, was prepared in which the promoter and N-terminal codons of bacterial genes may be fused to a portion of β-galactosidase, resulting in an easily scorable phenotype. Because transcription from the inserted promoter remains responsive to the host regulatory system, it is simple to screen mutagenized phage for isolates with aberrant regulatory phenotypes, and to determine the mutational changes by dideoxy sequence analysis. The feasibility of this method was demonstrated by identification of a large number of mutations in the regulatory regions of two genes, recA and lexA. Base substitutions that altered the phenotype of recombinant phage were identified both in the single LexA repressor binding site of recA and in the two binding sites of lexA, as well as in other sites that likely affect translational efficiency. My results suggest that this method will be generally useful for mutational analysis of transcriptional and translational regulatory elements. The mutants that were isolated by the above approach were used to investigate the specificity of LexA protein binding by quantifying the repressibility of a several mutant recA and lexA operator/promoter regions fused to the E. coli galactokinase (galK) gene. The results of this analysis indicated that two sets of four nucleotides (terminal nucleotide contacts), one set at each extreme end of the operator, are most critical for repressor binding. In addition, our results indicate that the repressor-operator interaction is symmetric in nature, in that mutations at symmetrically equivalent positions in the recA operator had comparable effects on repressibility. The inferred symmetry of the interaction justified the reevaluation of the consensus sequence by half-site comparison, which yielded the half-site consensus: (5') CTGTATAT. Although the first four positions of this half-site sequence have the greatest effect on LexA repressor binding, the last four are well conserved among binding sites and appear to modulate repressor affinity. The role of the terminal nucleotide contacts and the mechanism by which the internal sequences affect repressor binding is discussed.
Degree ProgramMolecular and Cellular Biology