Positive regulator of transcriptional elongation that is essential for cells to initiate differentiation. Interacts with RNA polymerase II and the Elongator complex and is required to sustain global levels of transcriptional elongation activity, specifically in differentiating tissues. Interacts with HAG3, NRPB3 and NRPB10L. Cytoplasmic in undifferentiated cells (meristem core) and nuclear in differentiating cells. Expressed in root and shoot apices and in leaf and flower primordia. Detected in the endosperm, embryo, meristems and in organ primordia, but not in mature cells. Found exclusively in the vascular bundles in mature leaves. No visible phenotype when heterozygous. Embryo lethal when homozygous. Over-expression of IYO activates premature cell differentiation and confers resistance to the elongation inhibitor 6-azauracil. Belongs to the RPAP1 family.