Sensory transducers of E. coli are encoded by homologous genes.

The tsr and tar genes, which are widely separated in the E. coli genome, encode functionally analogous transducer proteins that focus and integrate two distinct classes of chemosensory information. Physical mapping of these genes was achieved by use of transposon Tn5 mutagenesis of cloned DNA fragments. The polar effects of Tn5 insertions in the tar-cheR-cheB-cheY-cheZ region indicated that these genes are cotranscribed from a promoter upstream of tar and revealed the existence of a new gene, tap (taxis-associated protein), lying between tar and cheR. DNA hybridization studies demonstrated that the tsr gene possesses sequence homologies with the tar and tap genes, suggesting that they have all evolved from a common ancestor. The tap gene encoded a polypeptide of apparent molecular weight of 65,000, which may constitute a transducer protein of unknown specificity.