Frameshift revertant of Salmonella typhimurium producing histidinol dehydrogenase with a sequence of four extra amino acid residues

Abstract

Frameshift mutant hisD3018 of Salmonella typhimurium is a +1 type, most likely resulting from the insertion of an extra GC pair in DNA of the gene for histidinol dehydrogenase. R58 is an internally suppressed spontaneous revertant of hisD3018 which grows slowly in the absence of histidine. Histidinol dehydrogenase from R58 was found to contain a sequence of four extra residues when compared to the wild-type enzyme. This can be interpreted as resulting from a duplication of eleven bases, perhaps by formation of a loop during DNA repair or replication or by non-reciprocal recombination. Comparison of amino acid sequences in wild-type and R58 enzyme allows us to assign CCC and CCU as in vivo codons for proline and UGG for tryptophan and provides further evidence that the hisD3018 frameshift is expressed through inclusion of an extra C residue in mRNA. Quantitative analyses show that for a chemical subunit molecular weight of approximately 40,000 daltons, R58 histidinol dehydrogenase contains three tryptophan residues versus two for the wild-type enzyme. These mutational data support the molecular weight estimates for the chemical subunit made previously with the wild-type enzyme.