Transcriptional Regulation and Organization of thedcuA and dcuB Genes, Encoding Homologous Anaerobic C4-Dicarboxylate Transporters inEscherichia coli

ABSTRACT The dcuA and dcuB genes ofEscherichia coli encode homologous proteins that appear to function as independent and mutually redundant C4-dicarboxylate transporters during anaerobiosis. ThedcuA gene is 117 bp downstream of, and has the same polarity as, the aspartase gene (aspA), whiledcuB is 77 bp upstream of, and has the same polarity as, the anaerobic fumarase gene (fumB). To learn more about the respective roles of the dcu genes, the environmental and regulatory factors influencing their expression were investigated by generating and analyzing single-copy dcuA- anddcuB-lacZ transcriptional fusions. The results show thatdcuA is constitutively expressed whereas dcuBexpression is highly regulated. The dcuB gene is strongly activated anaerobically by FNR, repressed in the presence of nitrate by NarL, and subject to cyclic AMP receptor protein (CRP)-mediated catabolite repression. In addition, dcuB is strongly induced by C4-dicarboxylates, suggesting thatdcuB is under the control of an uncharacterized C4-dicarboxylate-responsive gene regulator. Northern blotting confirmed that dcuA (and aspA) is expressed under both aerobic and anaerobic conditions and thatdcuB (and fumB) is induced anaerobically. Major monocistronic transcripts were identified for aspA anddcuA, as well as a minor species possibly corresponding to an aspA-dcuA cotranscript. Five major transcripts were observed for dcuB and fumB: monocistronic transcripts for both fumB and dcuB; adcuB-fumB cotranscript; and two transcripts, possibly corresponding to dcuB-fumB and fumB mRNA degradation products. Primer extension analysis revealed independent promoters for aspA, dcuA, and dcuB, but surprisingly no primer extension product could be detected forfumB. The expression of dcuB is entirely consistent with a primary role for DcuB in mediating C4-dicarboxylate transport during anaerobic fumarate respiration. The precise physiological purpose of DcuA remains unclear.

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