The essential HupB and HupN proteins of Pseudomonas putida provide redundant and nonspecific DNA-bending functions

A protein mixture containing two major components able to catalyze a beta - recombination reaction requiring nonspecific DNA bending was obtained by fr actionation of a Pseudomonas putida extract. N-terminal sequence analysis a nd genomic data base searches identified the major component as an analogue of HupB of Pseudomonas aeruginosa and Escherichia coil, encoding one HU pr otein variant. The minor component of the fraction, termed HupN, was diverg ent enough from HupB to predict a separate DNA-bending competence. The dete rminants of the two proteins were cloned and hyperexpressed, and the gene p roducts were purified. Their activities were examined in vitro in beta -rec ombination assays and in vivo by complementation of the Hbsu function of Ba cillus subtilis. HupB and HupN were equally efficient in all tests, suggest ing that they are independent and functionally redundant DNA bending protei ns. This was reflected in the maintenance of in vivo activity of the sigma (54) Ps promoter of the toluene degradation plasmid, TOL, which requires fa cilitated DNA bending, in Delta hupB or Delta hupN strains. However, hupB/h upN double mutants were not viable. It is suggested that the requirement fo r protein-facilitated DNA bending is met in P. putida by two independent pr oteins that ensure an adequate supply of an essential cellular activity.