High Expression of the Second Lysine Decarboxylase Gene, ldc, in Escherichia coli WC196 Due to the Recognition of the Stop Codon (TAG), at a Position Which Corresponds to the 33th Amino Acid Residue of σ³⁸, as a Serine Residue by the Amber Suppressor, supD

抄録

  Escherichia coli WC196, which was obtained from the strain W3110 by nitrosoguanidine mutagenesis as an overproducer of lysine, produced approximately twenty times more cadaverine than did W3110, and had a twenty fold higher level of rpoS gene product, σ³⁸, than in W3110. Both WC196 and W3110 had a stop codon (TAG) in rpoS at position which corresponds to the 33th residue of σ³⁸ protein. In addition, WC196 but not W3110 had a mutation in the gene encoding Ser-tRNA (SerU), called, supD. Analysis of the amino acid sequence of a σ³⁸ preparation from WC196 showed that the 33th residue of σ³⁸ is a serine residue. The ΔrpoS ΔcadA mutant of E. coli W3110 harboring the plasmid containing rpoS, in which the TAG codon was converted to a TCG codon for serine-33 residue of σ³⁸, expressed a significant amount of Ldc and accumulated a large amount of σ³⁸. However, the ΔrpoS ΔcadA mutant of W3110 with the plasmid containing the intact rpoS from W3110 could synthesize neither σ³⁸ nor Ldc significantly.