σ(54), a vital protein for Myxococcus xanthus

The rpoN gene encoding the transcription factor σ(54) in Myxococcus xanthus has been cloned using a heterologous rpoN probe. The sequence of the cross-hybridizing DNA confirmed the existence of an ORF 1518 bp long that encodes a well conserved member of the σ(54) family of σ factors. Low- as well as high-stringency hybridizations detected only a single rpoN gene in the M. xanthus chromosome. In other bacteria, σ(54) is an alternative σ, and null mutants are viable. However, all attempts to construct a strain containing a null mutation in the M. xanthus rpoN have been unsuccessful. Partial diploids of rpoN(+)/rpoN null are viable. Recombination experiments with such partial diploids showed the impossibility of constructing, either by segregation or by transduction, a viable null haploid under any of a wide range of growth conditions. The product of the rpoN gene, σ(54), therefore appears to be essential for growth in M. xanthus.