Evidence for a posttranscriptional role of a TFIIICalpha-like protein in Chironomus tentans.

We have cloned and sequenced a cDNA that encodes for a nuclear protein of 238 kDa in the dipteran Chironomus tentans. This protein, that we call p2D10, is structurally similar to the alpha subunit of the general transcription factor TFIIIC. Using immunoelectron microscopy we have shown that a fraction of p2D10 is located at sites of transcription, which is consistent with a possible role of this protein in transcription initiation. We have also found that a large fraction of p2D10 is located in the nucleoplasm and in the nuclear pore complexes. Using gel filtration chromatography and coimmunoprecipitation methods, we have identified and characterized two p2D10-containing complexes that differ in molecular mass and composition. The heavy p2D10-containing complex contains at least one other component of the TFIIIC complex, TFIIIC-epsilon. Based on its molecular mass and composition, the heavy p2D10-containing complex may be the Pol III holoenzyme. The light p2D10-containing complex contains RNA together with at least two proteins that are thought to be involved in mRNA trafficking, RAE1 and hrp65. The observations reported here suggest that this new TFIIIC-alpha-like protein is involved in posttranscriptional steps of premRNA metabolism in Chironomus tentans.