A mutation in the largest subunit of RNA polymerase II alters RNA chain elongation in vitro.

An in vitro transcription system which utilized a semisynthetic DNA template (Kadesch, T. R., and Chamberlin, M. J. (1982) J. Biol. Chem. 257, 5286-5295) was developed and used to compare RNA chain elongation by wild type and mutant RNA polymerases II of Drosophila. With this template, all of the active polymerases rapidly initiated RNA chains at synthetic single-stranded sites at the ends of the DNA, and then entered a long (15 to 20 min) period of elongation through duplex regions of template before any measurable termination occurred. A comparison of wild type and mutant polymerase activities during this elongation phase indicated that a mutation to amanitin resistance reduces the rate at which the enzyme elongates transcripts. The reduced elongation rate of the mutant was associated with an altered substrate Km. Because the polymerase II mutation is in the largest enzyme subunit (Greenleaf, A. L. (1983) J. Biol. Chem. 258, 13403-13406), these results demonstrate a functional role for this subunit during RNA chain elongation.