The Bradyrhizobium japonicum fixBCX operon: identification of fixX and of a 5' mRNA region affecting the level of the fixBCX transcript.
The Bradyrhizobium japonicum fixX gene was identified and shown to be essential for symbiotic and free-living, microaerobic nitrogen fixation. The fixX gene encodes a ferredoxin-like protein which may be involved in a redox process (electron transport?) essential for nitrogenase activity. This gene was localized downstream of fixC and its expression was dependent on the fixB promoter, providing evidence for the existence of a fixBCX operon. Mutagenesis and sequence analysis of the unusually long, 709bp leader region between the fixB promoter and the fixB structural gene did not reveal the presence of a nif or fix gene that was absolutely essential for nitrogen fixation. However, a short open reading frame (ORF) within this region encoding a polypeptide of 35 amino acids (ORF35) was shown to be efficiently translated. Chromosomal deletion of a 400bp DNA fragment covering ORF35 resulted in a three-fold reduction of the fixBCX mRNA level, which in turn also reduced the nitrogen fixation activity of this mutant. This suggests a possible post-transcriptional control mechanism for the expression of the fixBCX operon involving the stabilization of fixBCX mRNA by ribosomes actively translating ORF35.
Gubler
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Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Zürich, Switzerland.
Zürcher
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Hennecke
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D000595:Amino Acid Sequence; D001483:Base Sequence; D015152:Blotting, Northern; D002874:Chromosome Mapping; D002876:Chromosomes, Bacterial; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics; D005288:Ferredoxins / Q000235:genetics; D005786:Gene Expression Regulation; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D009154:Mutation; D009586:Nitrogen Fixation / Q000235:genetics*; D009876:Operon; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics*; D012230:Rhizobiaceae / Q000235:genetics*; D014158:Transcription, Genetic; D001616:beta-Galactosidase / Q000235:genetics
D004269:DNA, Bacterial; D005288:Ferredoxins; D012333:RNA, Messenger; D001616:beta-Galactosidase