Non-viability of Haemophilus parasuis fur-defective mutants.

ABSTRACT

By complementation of an Escherichia coli fur mutant, the Haemophilus parasuis fur gene has been isolated from a genomic library of this organism. The H. parasuis fur gene is the distal one of a three-gene operon. Two genes placed upstream of the H. parasuis fur open-reading frame encode for a hypothetical protein and a flavodoxin, respectively. Attempts performed to isolate an H. parasuis fur-defective mutant either through manganese-resistance selection or exchange markers were unsuccessful. Likewise, anaerobic growth conditions do not enable the attainment of H. parasuis fur-defective mutants either. Nevertheless, H. parasuis clones carrying a knockout mutation in the chromosomal fur gene by insertion of a KmR cassette were obtained when a stable plasmid, containing an additional copy of the transcriptional unit to which the fur gene belongs, was present. Likewise, the presence of a plasmid in which the H. parasuis fur gene is under the control of the Escherichia coli tac promoter allows for the isolation of furKm mutants of this organism. Nonetheless, no fur-defective mutants may be isolated from H. parasuis cells harbouring a stable plasmid in which only the single fur gene is contained. These data clearly indicate that H. parasuis cell viability requires the presence of a wild-type fur gene.