Component of the Mediator complex, a coactivator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene-specific regulatory proteins to the basal RNA polymerase II transcription machinery. The Mediator complex, having a compact conformation in its free form, is recruited to promoters by direct interactions with regulatory proteins and serves for the assembly of a functional preinitiation complex with RNA polymerase II and the general transcription factors. Acts closely together with MAB12. Involved in the regulation of embryo patterning and cotyledon organogenesis. May act through transient repression of specific genes such as the ones responsive to auxin. Component of the Mediator complex. Interacts with CYCC1-2 (CDK8 homolog). A number of isoforms are produced. According to EST sequences. Ubiquitous. Highest expression in the shoot apex. Expressed in both the apical and basal cell at the one-cell embryo stage and in all cells of the embryo and suspensor through the globular stage. Decreases in the periphery of the hypocotyl and on the abaxial side of cotyledons from the heart stage to the torpedo stage, and by the bent cotyledon stage, restricted to the vascular tissue and the shoot and root apical meristems. Abnormal embryo and cotyledon development, and embryo lethality in most cases. When viable, plants are small with greatly delayed flowering time and sterile flowers. Belongs to the Mediator complex subunit 13 family. The predicted gene At1g55325 has been split into 2 genes: At1g55320 and At1g55325.