REPRESSION OF THE ESCHERICHIA-COLI MODABCD (MOLYBDATE TRANSPORT) OPERON BY MODE

The modABC gene products constitute the molybdate-specific transport s ystem in Escherichia coli. Another operon coding for two proteins whic h diverges from the modABCD operon has been identified. The first gene of this operon codes for a 262-amino-acid protein, designated ModE (2 8 kDa), and the second gene codes for a 490-amino-acid protein, ModF ( 54 kDa). The role of ModF has not yet been determined; however, mutati ons in modE derepressed modABCD transcription even in the presence of molybdate, suggesting that ModE is a repressor. ModE, in the presence of 1 mM molybdate, repressed the production of plasmid-encoded ModA an d ModB' proteins in an in vitro transcription-translation system. DNA mobility shift experiments confirmed that ModE binds to an oligonucleo tide derived from the operator region of the modABCD operon. Further e xperimentation indicated that ModE binding to target DNA minimally req uires an 8-bp inverted-repeat sequence, TAAC.GTTA. A highly conserved amino acid sequence, TSARNQXXG (amino acids 125 to 133), was identifie d in ModE and homologs from Azotobacter vinelandii, Haemophilus influe nzae, Rhodobacter capsulatus, and Clostridium pasteurianum. Mutants wi th mutations in either T or G of this amino acid sequence were isolate d as ''superrepressor'' mutants. These mutant proteins repressed modAB CD transcription even in the absence of molybdate, which implies that this stretch of amino acids is essential for the binding of molybdate by the ModE protein. These results show that molybdate transport in E. coli is regulated by ModE, which acts as a repressor when bound to mo lybdate.