RESUMEN
The nitrogenase structural genes (nifHDK) of the endophytic diazotroph Herbaspirillum seropedicae were isolated from a genomic bank by plate hybridization. Sequence analysis of the DNA showed a consensus promoter region upstream from the nifH gene containing a -24/-12 type promoter together with NifA- and integration host factor (IHF)- binding sites. The derived protein sequences of NifH, NifD and NifK contained conserved cysteine residues for binding iron-sulfur clusters and the iron-molybdenum cofactor. These protein sequences showed the strongest similarities to the nifHDK gene products of the symbiotic diazotroph Bradyrhizobium japonicum (93.5 per cent, 91.3 per cent and 83.3 per cent, respectively), the plant-associated diazotrophAzospirillum brasilense (90.0 per cent, 83.7 per cent and 75.1 per cent, respectively) and to Thiobacillus ferrooxidans (91.0 per cent, 83.4 per cent and 81.1 per cent, respectively) of the same phylogenetic group of the protobacteria.