SIII, also known as elongin, is a general transcription elongation factor that increases the RNA polymerase II transcription elongation past template-encoded arresting sites. Subunit A is transcriptionally active and its transcription activity is strongly enhanced by binding to the dimeric complex of the SIII regulatory subunits B and C (elongin BC complex). Heterotrimer of an A (ELOA, ELOA2 or ELOA3P), ELOB and ELOC subunit (By similarity). Interacts with ERCC6; the interaction is induced by DNA damaging agents or inhibitors of RNA polymerase II elongation (By similarity). Interacts (via BC-box) with CUL5 (By similarity). The BC-box, which mediates binding to the elongin BC complex, has the consensus sequence [APST]-L-x(3)-C-x(3)-[AILV].