these results strongly suggest that ( a ) the above-mentioned procedure is actually fractionating rna polymerase i into free and bound forms , ( b ) rna polymerase ib is the transcriptionally active form in vivo , ( c ) rna polymerase ib exists in excess in the nucleoli , but the amount of bound ib molecules , which are engaged in transcription in vivo , must be determined by some other factor ( s ) than the mere concentration of ib enzyme in the nucleolus , and ( d ) ia form is not an artifact of isolation but is always present in vivo at a certain amount , although the exact nature of this molecule is not known at present .
C0035681
rna polymerase
C0040649
transcription