FIS, AN ACCESSORIAL FACTOR FOR TRANSCRIPTIONAL ACTIVATION OF THE MAR (MULTIPLE ANTIBIOTIC-RESISTANCE) PROMOTER OF ESCHERICHIA-COLI IN THE PRESENCE OF THE ACTIVATOR MARA, SOXS, OR ROB

Transcription of the multiple antibiotic resistance marRAB operon incr eases when one of the sequence-related activators, MarA, SoxS, or Rob, binds to the ''marbox'' centered at -61.5 relative to the transcripti onal start site. Previous deletion analyses showed that an adjacent up stream ''accessory region'' was needed to augment the marbox-dependent activation. To analyze the roles of the marbox and accessory regions on mar transcription, thirteen promoters, each with a different 5-bp t ransversion of the -96 to -32 sequence, were synthesized, fused to lac Z, and assayed for beta-galactosidase production in single copy lysoge ns with appropriate genotypes. The accessory region is shown here to b e a binding site for Fis centered at -81 and to bind Fis, a small DNA- binding and -bending protein, with a K-d of approximate to 5 nM. The b inding of MarA to the marbox and that of Fis to its site were independ ent of each other. MarA, SoxS, and Rob each activated the mar promoter 1.5- to 2-fold when it had a wild-type marbox but Fis was absent. In the presence of MarA, SoxS, or Rob, Fis further enhanced the activity of the promoter twofold provided the promoter was also capable of bind ing Fis. However, in the absence of MarA, SoxS, or Rob or in the absen ce of a wild-type marbox, Fis nonspecifically lowered the activity of the mar promoter about 25% whether or not a wild-type Fis site was pre sent. Thus, Fis acts as an accessory transcriptional activates at the mar promoter.