text: Enzymes can catalyze up to several million reactions per second. Enzyme rates depend on solution conditions and substrate concentration. Like other enzymes POLR2 has a saturation curve and a maximum velocity (V). It has a K (substrate concentration required for one-half V) and a k (the number of substrate molecules handled by one active site per second). The specificity constant is given by k/K. The theoretical maximum for the specificity constant is the diffusion limit of about 10 to 10 (Ms), where every collision of the enzyme with its substrate results in catalysis. In yeast, mutation in the Trigger-Loop domain of the largest subunit can change the kinetics of the enzyme.
  Bacterial RNA polymerase, a relative of RNA Polymerase II, switches between inactivated and activated states by translocating back and forth along the DNA. Concentrations of [NTP] = 10 μM GTP, 10 μM UTP, 5 μM ATP and 2.5 μM CTP, produce a mean elongation rate, turnover number, of ~1 bp (NTP) for bacterial RNAP, a relative of RNA polymerase II.
  RNA polymerase II undergoes extensive co-transcriptional pausing during transcription elongation. This pausing is especially pronounced at nucleosomes, and arises in part through the polymerase entering a transcriptionally incompetent backtracked state. The duration of these pauses ranges from seconds to minutes or longer, and exit from long-lived pauses can be promoted by elongation factors such as TFIIS. In turn, the transcription rate influences whether the histones of transcribed nucleosomes are evicted from chromatin, or reinserted behind the transcribing polymerase.