DNA-BINDING PROPERTIES OF THE BETI REPRESSOR PROTEIN OF ESCHERICHIA-COLI - THE INDUCER CHOLINE STIMULATES BETI-DNA COMPLEX-FORMATION

The betT and betIBA genes govern glycine betaine synthesis from cholin e in Escherichia coli. In an accompanying paper me report that the bet T and berl promoters are divergently organized and partially overlappi ng and that both are negatively regulated by BetI in response to choli ne (T. Lamark, T. P. Rokenes, J. McDougall, and A. R. Strom, J. Bacter iol, 178:1655-1662, 1996). In this paper, we report that the in vivo s ynthesis rate of the BetI protein constituted only 10% of that of BetA and BetB dehydrogenase proteins, indicating the existence of a posttr anscriptional control of the betIBA operon. A genetically modified Bet I protein called BetI, which carries 7 extra N-terminal amino acids, was purified as a glutathione S-transferase fusion protein. Gel mobili ty shift assays showed that BetI formed a complex with a 41-bp DNA fr agment containing the -10 and -35 regions of both promoters. Only one stable complex was detected with the 41-bp fragment and all larger pro moter-containing fragments tested. In DNase I footprinting, BetI prot ected a region of 21 nucleotides covering both the -35 boxes. Choline stimulated complex formation but did not change the binding site of Be tI. We conclude that in vivo BetI is bound to its operator in both re pressed and induced cells and that BetI represents a new type of repre ssor.