The specific activity of yeast RNA polymerases A or B, when complexed with P37 cofactor, compares favorably with that of E. coli RNA polymerase. The stimulation is observed only with double-stranded DNA but does not result from DNase action. The Km for nucleotide substrates and the optimal conditions of transcription are not modified. P37 stimulates RNA synthesis by ternary transcription complexes in the presence of poly(rI) which prevents reinitiations. The RNA chain length, estimated by 5' end labeling or sedimentation, is increased in the presence of P37. On the other hand, the trinucleotide synthesis, which reflects the chain initiation reaction, is not affected. Therefore, the cofactor appears to act at the elongation step of RNA synthesis.
