Export of L-isoleucine from Corynebacterium glutamicum: a two-gene-encoded member of a new translocator family

Bacteria possess amino acid export systems, and Corynebacterium glutamicum excretes L-isoleucine in a process dependent on the proton motive force. In order to identify the system responsible for L-isoleucine export, we have used transposon mutagenesis to isolate mutants of C. glutamicum sensitive to the peptide isoleucyl-isoleucine. In one such mutant, strong peptide sensitivity resulted from insertion into a gene designated brnF encoding a hydrophobic protein predicted to possess seven transmembrane spanning helices. brnE is located downstream of brnF and encodes a second hydrophobic protein with four putative membrane-spanning helices. A mutant deleted of both genes no longer exports L-isoleucine, whereas an overexpressing strain exports this amino acid at an increased rate. BrnF and BrnE together are also required for the export of L-leucine and L-valine. BrnFE is thus a two-component export permease specific for aliphatic hydrophobic amino acids. Upstream of brnFE and transcribed divergently is an Lrp-like regulatory gene required for active export. Searches for homologues of BrnFE show that this type of exporter is widespread in prokaryotes but lacking in eukaryotes and that both gene products which together comprise the members of a novel family, the LIV-E family, generally map together within a single operon. Comparisons of the BrnF and BrnE phylogenetic trees show that gene duplication events in the early bacterial lineage gave rise to multiple paralogues that have been retained in alpha-proteobacteria but not in other prokaryotes analyzed.