In Klebsiella pneumoniae the products of six genes are essential for the biosynthesis of the nitrogenase cofactor (FeMo-co): nifH, nifQ, nifB, nijV, nifE and nifN (Kim, Rees 1992). Shah et al. (1999) demonstrated that the product of the nifX gene, enhanced the yield of FeMo-co in vitro. The nifENXorflorf2 gene cluster was found downstream from the nifHDK in Herbaspirillum seropedicae (Klassen 1999). Here we show that nifX and/or orfl orfl of H. seropedicae are required for nitrogenase activity under iron or molybdenum deficiency, suggesting the involvement of these genes in FeMo-co biosynthesis.
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