The fix Escherichia coli region contains four genes related to carnitine metabolism
✍ Scribed by Knut Eichler; Anne Buchet; Fabienne Bourgis; Hans-Peter Kleber; Marie-Andrée Mandrand-Berthelot
- Publisher
- John Wiley and Sons
- Year
- 1995
- Tongue
- English
- Weight
- 850 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0233-111X
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✦ Synopsis
Anaerobic carnitine metabolism in Escherichia coli was recently shown to involve six genes organized in the cai operon and located at the first minute on the chromosome. The DNA sequence lying at the 5' end of the cai locus was further investigated. It contains four open reading frames organized as an operon. In vivo overexpression of this DNA region revealed four polypeptides with apparent molecular masses of 27, 33, 45 and 6 kDa. These proteins displayed significant amino acid sequence homologies with polypeptides encoded by thefixABCX operons from Azorhizobium caulinodans and Rhizobium meliloti. The four ORFs were thus namedfixABCX. The first two gene products were also found to share a high degree of sequence similarity with the subunits / 3 and a, respectively, of mammalian electron transfer flavoproteins, suggesting a role for these proteins in a redox reaction. A singly polycistronic 5 kb mRNA transcript was detected in Northern blots under anaerobic conditions in the presence of DL-carnitine. Expression of afixA-ZacZ transcriptional fusion was induced by L(-)-carnitine and crotonobetaine but not by D(+)-carnitine, y-butyrobetaine, glycinebetaine and choline as found previously for the carnitine pathway. Similarly, thefix operon was repressed by glucose and nitrate. Moreover, expression of thefix operon was induced by the global regulatory proteins CRP and FNR and repressed by the histone-like protein H-NS. All these regulatory proteins have been shown also to control expression of carnitine enzymes. Results from Northern blots and lacZ fusion studies indicate a common regulation of expression offix and cai operons, which implies a physiological linkage between these two loci.
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