SPECIFIC INTERACTION OF IHF WITH RIBS, A CLASS OF BACTERIAL REPETITIVE DNA ELEMENTS LOCATED AT THE 3' END OF TRANSCRIPTION UNITS

The prokaryotic REP (repetitive extragenic palindromes) or PU (palindromic units) sequences are often associated with other repetitive elements, forming arrangements which have been called 'BIMEs' (bacterial interspersed mosaic elements). It is estimated that the Escherichia coli chromosome carries approximately 300 - 500 BIMEs, whose biological role is at present unknown. We have identified a family of BIMEs consisting of two converging REP sequences flanking a 35 bp conserved segment which carries a static DNA bend and a binding site for IHF, the integration host factor of E. coli. We estimate that the E. coli genome harbors approximately 100 copies of this module, which we name 'RIB' (reiterative ihf BIME). We have analyzed by gel retardation and by footprinting the in vitro interaction of IHF with individual RIBs, and shown that the protein binds strongly and specifically to their center. We have also demonstrated binding of IHF to the chromosomal population of RIBs, using a new approach which combines two-dimensional bandshift and Southern blotting. RIB elements are at the end of transcription units, and thus define a new class of ihf sites. Possible implications for genome structure and DNA topology are discussed.