Transcribing RNA polymerase II is phosphorylated at CTD residue serine-7

RNA polymerase II is distinguished by its large carboxyl- terminal repeat domain ( CTD), composed of repeats of the consensus heptapeptide Tyr(1)-Ser(2)-Pro(3)-Thr(4)-Ser(5)-Pro(6)-Ser(7). Differential phosphorylation of serine-2 and serine-5 at the 5 ' and 3 ' regions of genes appears to coordinate the localization of transcription and RNA processing factors to the elongating polymerase complex. Using monoclonal antibodies, we reveal serine-7 phosphorylation on transcribed genes. This position does not appear to be phosphorylated in CTDs of less than 20 consensus repeats. The position of repeats where serine-7 is substituted influenced the appearance of distinct phosphorylated forms, suggesting functional differences between CTD regions. Our results indicate that restriction of serine-7 epitopes to the Linker- proximal region limits CTD phosphorylation patterns and is a requirement for optimal gene expression.