STUDIES OF THE REGULATION OF THE ILVGMEDA OPERON IN SERRATIA MARCESCENS
Abstract
The nucleotide sequence preceding the ilvGMEDA operon of Serratia marcescens strain SU3 (wild type) has been examined and compared with that of Escherichia coli K-12. Differences between these two leader sequences are extensive. Different secondary structures had to be postulated for the leader transcript of S. marcescens. In S. marcescens, the ilv leader region specifies a putative peptide containing only a single leucine codon that could be involved in leucine-mediated control by attenuation. A plasmid ((,p)PU134) containing the DNA of the S. marcescens ilv control region and the ilvGMED genes was studied as a single chromosomal copy in E. coli strain auxotrophic for all three branched-chain amino acids. The S. marcescens ilv genes responded to a multivalent control similar to that found in other enteric organisms. Furthermore, the S. marcescens ilv genes were depressed when the charging of leucine tRNA was restricted in a leuS derivative of E. coli that had been transformed with (,p)PU134. It was concluded that ribosome stalling leading to deattenuation is not dependent on either tandem or a consecutive series of codons for the regulatory amino acid. However, the fact that the single leucine codon is a less frequently used codon (CUA) may be important. In plasmid (,p)PU253, the DNA giving rise to the terminator has been deleted by a site-specific deletion. The observation that the expression of the ilv genes of (,p)PU253 is no longer subject to multivalent repression indicates that attenuation is the predominant if not the sole mechanism in the endproduct-specific regulation of the expression of the ilv operon of S. marcescens. S. marcescens mutant strain SU4 exhibited a nearly constitutive ilv operon expression phenotype. Nucleotide sequence analysis of the ilv leader region of SU4 indicated that at the terminator region there is a G (--->) A single base change from strain SU3 (wild type) to strain SU4. The single base change could reduce the stability of the terminator of SU4 greatly. In vivo studies indicated that the single base change accounts for the constitutive ilv operon expression phenotype of the mutant.
Degree
Ph.D.
Subject Area
Biology
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