Cloning and nucleotide sequence of the brnQ gene, the structural gene for a membrane-associated component of the LIV-II transport system for branched-chain amino acids in Salmonella typhimurium.

The genetically defined gleR-brnQ region responsible for the branched-chain amino acid transport in Salmonella typhimurium was mapped in the 3.3-kilobase SalI-PstI segment of plasmid pOH56 by complementation analysis. By subcloning and genetic recombination analysis, the gleR and brnQ3 mutational sites were localized within the 0.85-kilobase SalI-EcoRV segment, and brnQ4 within the 0.8-kilobase EcoRV-HindIII segment. The nucleotide sequence of the brnQ gene and its flanking regions was determined. The brnQ gene is encoded by the sequence starting 24 base pairs upstream from the EcoRV site. Transcription of the brnQ gene starts at three sites separated by 171, 173 and 174 nucleotides, respectively, from the initiation codon. The promoter sequences can be seen in the immediate upstream region of the transcription initiation sites. There is a long silent region between the transcription initiation sites and a potential Shine-Dalgarno nucleotide sequence. The coding sequence of the brnQ gene, which is 1317 base pairs long, specifies a very hydrophobic protein of 439 amino acid residues.