Isoform 1 appears to be stable component of the Pol II(G) complex form of RNA polymerase 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. Isoform 1 may play a role in Mediator-dependent regulation of transcription activation. Isoform 1 acts in vitro as 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 (By similarity). Isoform 1 is a component of the Pol II(G) complex, which contains the RNA polymerase II (Pol II) core complex subunits and Polr2m isoform 1 and appears to be an abundant form of Pol II. Belongs to the GRINL1 family.