Transcriptional regulation of the aconitase genes (acnA and acnB) of Escherichia coli

Escherichia coli contains two differentially regulated aconitase genes, acnA and acnB. Two acnA promoters transcribing from start points located 407 bp (P1(acnA)) and 50 bp (P2(acnA)) upstream of the acnA coding region, and one acnB promoter (P-acnB) with a start point 95 bp upstream of the acnB coding region, were identified by primer extension analysis. A 2.8 kb acnA monocistronic transcript was detected by Northern blot hybridization, but only in redox-stressed (methyl-viologen-treated) cultures, and a 2.5 kb acnB monocistronic transcript was detected in exponential-but not stationary-phase cultures. These findings are consistent with previous observations that acnA is specifically subject to SoxRS-mediated activation, whereas acnB encodes the major aconitase that is synthesized earlier in the growth cycle than AcnA. Further studies with acn-lacZ gene fusions and a wider range of transcription regulators indicated that acnA expression is initiated by sigma(38) from P1(acnA), and from P2(acnA) it is activated directly or indirectly by CRP, FruR, Fur and SoxRS, end repressed by ArcA and FNR. In contrast, acnB expression is activated by CRP and repressed by ArcA, FruR and Fis from P-acnB. Comparable studies with fum-lacZ fusions indicated that transcription of fumC, but not of furnA or fumB, is initiated by RNA polymerase containing sigma(38). It is concluded that AcnB is the major citric acid cycle enzyme, whereas AcnA is an aerobic stationary-phase enzyme that is specifically induced by iron and redox-stress.