We report the isolation and characterization of a mutant of unable to grow aerobically on non-fermentable substrates, except for very slow growth on glycerol. The mutant contains cytochrome oxidases and , and grows anaerobically with alternative electron acceptors. Oxygen consumption rates of cell-free extracts were low relative to activities in an isogenic control strain, but were restored by adding ubiquinone-1 to cell-free extracts. Transformation with a cloned 2.8 kb I-RV fragment of chromosomal DNA restored the ability of this mutant (AN2571) to grow on succinate and also restored cellular quinone levels in this strain. The plasmid also complemented a previously isolated mutant (AN151) for aerobic growth on succinate. The nucleotide sequence revealed a 0.7 kb portion of A. Unidirectional nested deletions from this fragment and complementation analysis identified an open reading frame encoding a protein with a predicted molecular mass of 26.5 kDa. This gene () encodes the enzyme 2-octaprenyl-3-methyl-5-hydroxy-6-methoxy-1,4-benzoquinone methyltransferase, which catalyses the terminal step in the biosynthesis of ubiquinone. The open reading frame is preceded by a putative Shine-Dalgarno sequence and followed by three palindromic unit sequences. Comparison of the inferred amino acid sequence of UbiG with the sequence of other -adenosylmethionine (AdoMet)-dependent methyltransferases reveals a highly conserved AdoMet-binding region. The cloned 2.8 kb fragment also contains a sequence encoding the C-terminus of a protein with 42–44% identity to fungal acetyl-CoA synthetases.


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