An 800 bp AccI-PUCII DNA containing promoter region of nodABC (P1) from A. caulinodans ORS571 was cloned and used as a hybridization probe against the chromosome DNA, which led to the identification of another 8.4-kb EcoRI fragment showing homology to P1. A corresponding clone of 8.4-kb DNA was isolated from a pLAFI gene bank (pRG90). The restriction enzyme analysis and DNA-DNA hybridization of 8.4-kb DNA indicated the P1 homology was located in the 450-bp SmaI-SphI region (P2 region). omega insertion deletion of 8.4 kb DNA resulted in a mutant strain ORS571-5 that delayed to nodulate the stems of S. rostrata. This phenotype was complemented by the introduction of pRK84 carrying nod locus 5 DNA.