[Isoform 1]: Appears to be a stable component of the Pol II(G) complex form of RNA polymerase II (Pol II). Pol II synthesizes mRNA precursors and many functional non-coding RNAs and is the central component of the basal RNA polymerase II transcription machinery. May play a role in the Mediator complex-dependent regulation of transcription activation. Acts as a negative regulator of transcriptional activation; this repression is relieved by the Mediator complex, which restores Pol II(G) activator-dependent transcription to a level equivalent to that of Pol II. {ECO:0000269|PubMed:16769904, ECO:0000269|PubMed:30190596}. This is the function of POLR2M (RNA polymerase II subunit M, Ensembl gene identifier ENSG00000255529).