1887

Abstract

NifA, the transcriptional activator of nitrogenase () genes, has up to now been described to be regulated in its activity via the sensor NifL only for members of the γ-subgroup of the . This paper reports a functionally similar NifL-like protein outside this group in sp. strain BH72, a diazotrophic grass endophyte belonging to the β-subgroup of the . Its structural genes for nitrogenase () are regulated in response to combined nitrogen and O and expressed endophytically inside rice roots. In order to characterize nitrogen-regulatory genes, an sp. BH72 genomic library was used to select cosmids that complemented a mutation in . Sequence analysis of the 34 kb genomic region complementing showed two ORFs with sequence identities of 44% to NifL and 61% to NifA of . According to Northern blot and reverse transcriptase PCR analysis, the transcript was more abundant at low combined nitrogen and O levels, results which were corroborated by GUS (β-glucuronidase) assays using a transcriptional :: fusion. N fixation was abolished in a NifLA and a NifA mutant, wild-type fixation being restored by . The NifLA mutant also failed to activate :: expression, indicating that NifA is the obligate transcriptional activator for . A mutant was diazotrophic and did not show repression of :: by ammonium or O, suggesting that NifL of sp. strain BH72 has a similar role in inactivating NifA in response to O and combined nitrogen as NifL in bacteria of the γ-.

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2002-10-01
2019-12-12
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