DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase I which synthesizes ribosomal RNA precursors. Isoform 1 is involved in UBTF-activated transcription, presumably at a step following PIC formation. Isoform 2 has been described as a component of preformed T-cell receptor (TCR) complex. Component of the RNA polymerase I (Pol I) complex consisting of at least 13 subunits. Interacts with TAF1A thereby associates with the SL1 complex. Interacts with UBTF. Interacts with POLR1E/PRAF1 through its N-terminal region (By similarity). Isoform 2 interacts with CD3E. Found at the fibrillar centers of the nucleolus in interphase and during cell division it is localized to the nucleolus organizer regions of the chromosomes. Isoform 2 undergoes tyrosine phosphorylation upon T-cell receptor (TCR) stimulation. This phosphorylation has not been confirmed by other groups. Isoform 1 is phosphorylated on tyrosine residues in initiation-competent Pol I-beta complexes but not in Pol I-alpha complexes. It is in an antisense orientation to and overlaps the gene of the DNA repair enzyme ERCC1. This gene overlap is conserved in mouse, suggesting an important biological function. Has sharply different functional characteristics. Belongs to the eukaryotic RPA34 RNA polymerase subunit family. It is not known whether the so-called human ASE1 and human CAST proteins represent two sides of a single gene product with sharply different functional characteristics. Experiments done with the mouse homolog protein are in favor of an implication of this gene in rRNA transcription instead of T-cell receptor signaling.