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If Mg 2+  ion is replaced by Mn 2+  ion, RNA polymerase tends to misincorporate noncognate nucleotide, which is thought to be one of the reasons for the toxicity of Mn 2+  ion. Therefore, most cells have Mn 2+  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 Mn 2+  ion, we compared the properties of cyanobacterial and  E. coli  RNA polymerases. The cyanobacterial enzyme showed a lower level of abortive transcription and less misincorporation than the  E. coli  enzyme. 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 Mn 2+  ion by deliberate incorporation of a nucleotide at the cost of the elongation rate. The cyanobacterial and the  E. coli  enzymes showed different sensitivities to Mg 2+  ion, and the physiological role of the difference is also discussed.

Comparative Study of Cyanobacterial and E. coli RNA Polymerases: Misincorporation, Abortive Transcription, and Dependence on Divalent Cations