Histone-like protein HU and bacterial DNA topology: suppression of an HU deficiency by gyrase mutations.

The abundant bacterial protein called HU has the ability to wrap and bend DNA in vitro, and thus it has long been thought to play a role in DNA supercoiling. In the absence of HU, Escherichia coli formed tiny colonies on agar, rapidly accumulated suppressor mutations, and was hypersensitive to novobiocin. Three types of evidence implicated gyrase in the suppression of an HU deficiency. First, spontaneous suppressors that restored normal growth and reduced sensitivity to novobiocin mapped in gyrB, one of the genes encoding DNA gyrase. Second, a pair of known gyrB mutations (gyrB-203 Ts gyrB-221 NovR) allowed normal growth at permissive (30 degrees C) but not at intermediate (37 degrees C) conditions. Third, introduction of a gyrB-expressing plasmid restored normal colony size. DNA supercoiling comparisons showed that chromosomal supercoiling decreased in the absence of HU and increased toward wild-type levels in the presence of a spontaneous gyrB suppressor. Taken together, these data establish that HU has a physiological role in chromosomal DNA topology, probably by facilitating the action of gyrase.