Synergistic effect of himA and gyrB mutations: evidence that him functions control expression of ilv and xyl genes.

We have constructed Escherichia coli strains containing mutations at two different loci, both originally selected for failure to support lambda site-specific recombination: himA and gyrB-him(Ts). Although the gyrB-him(Ts) mutations by themselves reduce supercoiling at high temperature, the double mutants show a far greater effect on supercoiling. Our studies show that growth of phage lambda is severely inhibited and that maintenance of plasmid pBR322 is extremely unstable in the double mutants. Physiological studies also reveal that the double mutants are isoleucine auxotrophs at 42 degrees C. The fact that himA mutants are isoleucine auxotrophs at 42 degrees C in the presence of leucine suggests that a significant component of the isoleucine auxotrophy of the double mutants is a result of the himA mutation. The himA gene encodes the alpha subunit of a protein called the integration host factor. Since mutations in the hip or himD gene encoding beta, the other subunit of the integration host factor, also result in isoleucine auxotrophy in the presence of leucine, we suggest that the integration host factor regulates the synthesis of at least one of the enzymes in the ilv pathway, acetohydroxyacid synthase I, which is encoded by the ilvB gene. Studies of the utilization of various sugars as the sole carbon source suggest that the integration host factor controls expression of some gene(s) involved in the utilization of xylose.