Identification and characterization of the new gene rhtA involved in threonine and homoserine efflux in Escherichia coli.

The rhtA gene known as the ybiF ORF in the genome of Escherichia coli was identified as a new gene involved in threonine and homoserine efflux. This gene encodes a highly hydrophobic membrane protein that contains 10 predicted transmembrane segments. The rhtA23 mutation, which is an A-for-G substitution at position -1 in relation to the ATG start codon, increases the expression level of the rhtA gene. The overexpression of rhtA gene results in resistance to inhibitory concentrations of homoserine, threonine and a variety of other amino acids and amino acid analogues, reduced threonine and homoserine accumulation in resistant cells and increased production of threonine, homoserine, lysine and proline by the respective producing strains. The RhtA protein belongs to a vast family of transporters. The genome of E. coli contains at least 10 paralogues of RhtA. Phylogenetic analysis indicates that a common ancestor of living organisms contained several RhtA homologues.