STRUCTURAL AND FUNCTIONAL DIVERSITY AMONG BACTERIAL INTERSPERSED MOSAIC ELEMENTS (BIMES)

Palindromic units (PU or REP) were defined as 40-nucleotide DNA sequen ces which are highly repeated in the genome of several members of the Enterobacteriaceae. They were shown to be a constituent of the bacteri al interspersed mosaic element (BIME), in which they are associated wi th other repetitive sequences. We report here that Escherichia coli PU sequences contain three motifs (Y, Z1 and Z2), leading to the definit ion of two BIME families. The BIME-1 family, highly conserved over 145 nucleotides, contains two PUs (motifs Y and Z1). The BIME-2 family co ntains a variable number of PUs (motifs Y and Z2). We present evidence , using band shift experiments, that each PU motif binds DNA gyrase wi th a different affinity. This suggests that the two families are funct ionally distinct.