Bacterial chemotactic-signal transducers [1,2] are proteins that respond to changes in the concentration of attractants and repellents in the environment, and transduce a signal from the outside to the inside of the cell. These proteins undergo two covalent modifications: deamidation and reversible methylation. Attractants increase the level of methylation while repellents decrease it. The methyl groups are added by the methyl-transferase cheR and are removed by the methylesterase cheB.

In Escherichia coli and related bacteria, there are four different chemotactic transducers:

• tsr, responsible for taxis to the attractants L-serine and related amino acids and away from the repellents leucine, indole, and weak acids.
• tar, responsible for taxis to the attractants L-aspartate and related amino and dicarboxylic acids. Also mediates taxis to the attractant maltose via an interaction with the periplasmic maltose-binding protein. Mediates taxis away from the repellents cobalt and nickel.
• trg, responsible for taxis to ribose and galactose via an interaction with the periplasmic ribose- or galactose-binding proteins.
• tap, responsible for taxis towards dipeptides via an interaction with the periplasmic dipeptide-binding protein.

In Enterobacter aerogenes [3] there are at least two different chemotactic transducers:

• tsr, responsible for taxis to the attractant L-serine.
• tas, responsible for taxis to the attractant L-aspartate.

In Salmonella typhimurium [4], in addition to tar, there is:

• tcp, responsible for taxis to the attractant citrate.

All these proteins are composed of the same structural domains: a N-terminal region that resembles a signal peptide, but which is not removed from the mature protein and serves as a membrane-spanning region; a periplasmic domain of about 160 amino acids that forms the receptor domain; a second transmembrane region and finally a C-terminal cytoplasmic domain of about 300 amino acids which contains the methylation sites.

The methyl-accepting sites are specific glutamate residues (some of these sites are translated as glutamine but are irreversibly deamidated by cheB). They are clustered in two regions of the cytoplasmic domain [5]. As a signature pattern we have selected the first of these two regions.