Comparative Study of Cyanobacterial andE. coliRNA Polymerases: Misincorporation, Abortive Transcription, and Dependence on Divalent Cations

Abstract

If Mg2+ion is replaced by Mn2+ion, RNA polymerase tends to misincorporate noncognate nucleotide, which is thought to be one of the reasons for the toxicity of Mn2+ion. Therefore, most cells have Mn2+ion at low intracellular concentrations, but cyanobacteria need the ion at a millimolar concentration to maintain photosynthetic machinery. To analyse the mechanism for resistance against the abundant Mn2+ion, we compared the properties of cyanobacterial andE. coliRNA polymerases. The cyanobacterial enzyme showed a lower level of abortive transcription and less misincorporation than theE. colienzyme. Moreover, the cyanobacterial enzyme showed a slower rate of the whole elongation by an order of magnitude, paused more frequently, and cleaved its transcript faster in the absence of NTPs. In conclusion, cyanobacterial RNA polymerase maintains the fidelity of transcription against Mn2+ion by deliberate incorporation of a nucleotide at the cost of the elongation rate. The cyanobacterial and theE. colienzymes showed different sensitivities to Mg2+ion, and the physiological role of the difference is also discussed.