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

Abstract

We have constructed Escherichia coli strains containing mutations at two different loci, both originally selected for failure to support X site-specific recombination: himA and gyrB-him(Ts). Although the gyrBhim(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 A 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°C. The fact that himA mutants are isoleucine auxotrophs at 42°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 ao subunit of a protein called the integration host factor. Since mutations in the hip or himD gene encoding ,B, 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.An essential step in lysogeny by coliphage X is the integration of the phage genome into the bacterial chromosome. Integration occurs by a site-specific recombination reaction between a unique site on the phage genome (attP) and a unique site on the bacterial chromosome (attB) (reviewed in references 22 and 27a). This site-specific recombination reaction can proceed in the absence of both the phage Red and host Rec systems for generalized recombination.

Biology of bacterial deoxyribonucleic acid topoisomerases.

“…In a himA deletion mutant, gyrA expression is reduced about fourfold whereas gyrB expression is unchanged (40). It is interesting to note that supercoiling activity in the himA deletion mutant, as measured by introduction of titratable supercoils into superinfecting bacteriophage lambda, is the same as that in wild-type strains (35,40). This finding supports the idea discussed above that the concentration of the gyrB protein is more important than the concentration of the gyrA protein in controlling supercoiling.…”

“…This finding supports the idea discussed above that the concentration of the gyrB protein is more important than the concentration of the gyrA protein in controlling supercoiling. The himA deletion, presumably through its effect on lowering production of gyrA protein, does have an effect on gyrase activity if placed in a strain having a temperature-sensitive gyrB mutation: supercoiling activity, measured with superinfecting bacteriophage lambda in cells incubated at restrictive temperature, is lower than that in cells having only the gyrB mutation (35,40). b +, Increase; 0, no change; -, decrease; ND, not determined.…”

Integration host factor ofEscherichia coliregulates early- and repressor transcription of bacteriophage Mu by two different mechanisms

“…Besides in integration of the lambda genome IHF has been found to participate in a variety of other processes such as: positive and negative control of transcription of several E.coli and phage genes (14)(15)(16), replication of plasmid pSC101 (17), transposition of IS1 (18), encapsidation of lambdoid phage 21 (19) and post-transcriptional control of cII expression Of bacteriophage lambda (20). Until now the mechanism by which IHF is involved in these processes is unclear.…”

Control of Directionality in Lambda Site Specific Recombination

“…highlighted by the isolation of Int mutants capable of carrying out recombination in the absence of IHF (12,13). In addition to its role in recombination, IHF is involved in the regulation of several bacterial processes and mutants display pleiotropic deficiencies (14)(15)(16). The binding of these proteins to attP results in the formation of a discrete protein-DNA complex ("intasome") that can be visualized in the electron microscope (17); and, on the basis of nuclease protection experiments and topological analyses of recombination, this complex appears to have DNA wrapped around or along the protein core with the same handedness as that of a nucleosome (18,19).…”