Researchers artificially mutated a serine to an alanine in the C-terminal domain (CTD) of RNA polymerase II. In their in vitro transcription system, they found that this mutated polymerase could no longer transcribe. What is the best hypothesis to explain this observation? A. Without the serine, Pol II was no longer able to bind the DNA template, and therefore no transcription could occur.
B. The serine is important for interaction with sigma factors; without the sigma factor binding properly, transcription could not initiate.
C. The serine is important for the methylation of CTD in order to form a pre- initiation complex; without the methylation, no transcription could occur.
D. The serine is important for the phosphorylation of CTD necessary for transcription elongation; without it no transcription could occur.