UNTRANSLATED SEQUENCE UPSTREAM OF MARA IN THE MULTIPLE ANTIBIOTIC-RESISTANCE LOCUS OF ESCHERICHIA-COLI IS RELATED TO THE EFFECTOR-BINDING DOMAIN OF THE XYLS TRANSCRIPTIONAL ACTIVATOR

MarA, the 129-amino-acid (aa) protein which plays a crucial role in th e multiple antibiotic resistance (Mar) phenotype in Escherichia coli, shows homology to members of the XylS/AraC family of transcriptional r egulators. Although these proteins vary in size from around 100 to 350 aa they all contain a DNA-binding domain with a helix-turn-helix moti f. The larger ones, e.g., XylS, AraC, and Rob, contain an additional d omain either at their amino- or at their carboxy-terminus. This domain is important for effector-binding or dimerization or of unknown funct ion. MarA consists only of the DNA-binding component. Nevertheless, a sequence with a coding potential of 141 aa upstream of its ATG start-c odon showed 20.5-26.9% aa identity with the corresponding section with in the effector-binding domain of XylS from the TOL plasmid of Pseudom onas putida when translated in the same reading frame as MarA. However , the reading frame was interrupted by 11 translational stops. In anot her frame, this upstream sequence actually codes for a real protein, M arR, that is completely unrelated to XylS. Implications for the putati ve evolution of regulatory proteins through translocation of domains f ollowed by adaptation are discussed.