It has been shown [1] that some proteins involved in the transport of aromatic amino acids in Escherichia coli and related bacteria are evolutionary related; these permeases are:

• Tryptophan-specific transport protein (tryptophan permease) (gene mtr).
• Low affinity tryptophan permease (gene tnaB or trpP).
• Tyrosine-specific transport protein (tyrosine permease) (gene tyrP).
• Tyrosine permease from Erwinia herbicola (gene: tutB).

These permeases are proteins of about 400 to 420 amino acids which probably contain 11 or 12 transmembrane regions. The best conserved domain is a stretch of 20 residues which seems to be located in a cytoplasmic loop between the first and second transmembrane region.