contains two differentially regulated aconitase genes, and Two promoters transcribing from start points located 407 bp ( ) and 50 bp ( ) upstream of the coding region, and one promoter ( ) with a start point 95 bp upstream of the coding region, were identified by primer extension analysis. A 2.8 kb monocistronic transcript was detected by Northern blot hybridization, but only in redox-stressed (methyl-viologen-treated) cultures, and a 2.5 kb monocistronic transcript was detected in exponential- but not stationary-phase cultures. These findings are consistent with previous observations that is specifically subject to SoxRS-mediated activation, whereas encodes the major aconitase that is synthesized earlier in the growth cycle than AcnA. Further studies with gene fusions and a wider range of transcription regulators indicated that expression is initiated by σ from and from it is activated directly or indirectly by CRP, FruR, Fur and SoxRS, anc repressed by ArcA and FNR. In contrast, expression is activated by CRP and repressed by ArcA, FruR and Fis from Comparable studies with fusions indicated that transcription of but not of or is initiated by RNA polymerase containing σ 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.


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