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ThedcuA anddcuB 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 thedcu genes, the environmental and regulatory factors influencing their expression were investigated by generating and analyzing single-copydcuA - anddcuB-lacZ transcriptional fusions. The results show thatdcuA is constitutively expressed whereasdcuB expression is highly regulated. ThedcuB 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 thatdcuA (andaspA ) is expressed under both aerobic and anaerobic conditions and thatdcuB (andfumB ) is induced anaerobically. Major monocistronic transcripts were identified foraspA anddcuA , as well as a minor species possibly corresponding to anaspA-dcuA cotranscript. Five major transcripts were observed fordcuB andfumB : monocistronic transcripts for bothfumB anddcuB ; adcuB-fumB cotranscript; and two transcripts, possibly corresponding todcuB-fumB andfumB mRNA degradation products. Primer extension analysis revealed independent promoters foraspA ,dcuA , anddcuB , but surprisingly no primer extension product could be detected forfumB . The expression ofdcuB 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.

journal of bacteriology

Transcriptional Regulation and Organization of the dcuA and dcuB Genes, Encoding Homologous Anaerobic C 4 -Dicarboxylate Transporters in Escherichia coli