Nucleotide sequences and genomic constitution of five tryptophan genes of Lactobacillus casei

Abstract

Five trp genes, trpD, trpC, trpF, trpB, and trpA, of Lactobacillus casei were cloned by transformation of tryptophan auxotrophic mutants of the respective trp genes in Escherichia coli. These trp genes appear to constitute an operon and are located in the above order in a segment of DNA of 6,468 base pairs. The entire nucleotide sequence of this DNA segment was determined. Five contiguous open reading frames in this segment can encode proteins consisting of 341, 260, 199, 406, and 266 amino acids, respectively, in the same direction. The amino acid sequences of these proteins exhibit 25.5-50.2% homology with the amino acid sequences of the corresponding trp enzymes of E. coli. Two trp genes, trpC and trpF, from L. casei can complement mutant alleles of the corresponding genes of E. coli. However, neither the trpA gene nor the trpB gene of L. casei can complement mutations in the E. coli trpA gene and the trpB gene, respectively, suggesting that the protein products of the L. casei and E. coli trpA and trpB genes, respectively, cannot form heterodimers of tryptophan synthetase with activity. Other features of the coding and flanking regions of the trp genes are also described.