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The Bradyrhizobium japonicum fixBCX operon: identification of fixX and of a 5’mRNA region affecting the level of the fixSCX transcript
Author(s) -
Gubler M.,
Zürcher T.,
Hennecke H.
Publication year - 1989
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/j.1365-2958.1989.tb01803.x
Subject(s) - operon , biology , open reading frame , gene , mutant , nitrogenase , genetics , microbiology and biotechnology , structural gene , bradyrhizobium japonicum , messenger rna , ribosome , peptide sequence , nitrogen fixation , rna , rhizobiaceae , bacteria , symbiosis
Summary The Bradyrhizobfum 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, 709 bp 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 fixSCX 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.