Physical organization of phytobeneficial genes nifH and ipdC in the plant growth-promoting rhizobacterium Azospirillum lipoferum 4VI

The physical organization of phytobeneficial genes was investigated in the plant growth-promoting rhizobacterium Azospirillum lipoferum 4VI by hybridization screening of a bacterial artificial chromosome (BAC) library. Pulsed-field gel electrophoresis gave an estimated 5.7-Mb genome size for strain 4VI and a coverage level of 9 for the BAC library. The phytobeneficial genes nifH (associative nitrogen fixation) and ipdC (synthesis of the phytohormone indoleacetic acid) are chromosomal, but no BAC clone containing both genes was found, pointing to the absence of any genetic island containing nifH and ipdC. A 11.8-kb fragment containing nifH was analyzed. Neighboring genes implicated in nitrogen fixation (nifH, draT, draG) or not (arsC, yafJ and acpD) were organized as in A. brasilense. In contrast, the region located downstream of acpD contained four housekeeping genes (i.e. genes encoding DapF-, MiaB- and FtsY-like proteins, as well as gene amn) and differed totally from the one found in A. brasilense.