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. Author manuscript; available in PMC: 2020 Aug 1.
Published in final edited form as: Mol Microbiol. 2019 Jun 3;112(2):335–347. doi: 10.1111/mmi.14309

Fig. 1. DNA and RNA synthesis up to the discovery of σ.

Fig. 1.

Early studies of DNA and RNA synthesis were enabled by biochemical approaches, including the purification and characterization of the enzymes and their substrate requirements. Unlike DNA polymerase, RNA polymerase does not require a pre-existing primer for synthesis, and can recognize start sites (promoters). This latter activity requires the σ subunit, discovered in 1969 (inset is adapted from Burgess et al., 1969).