Cells employ transcription-coupled repair (TCR) to eliminate transcription-blocking DNA lesions. The binding of the TCR-specific repair factor CSB triggers DNA damage-induced ubiquitylation of RNA polymerase II (RNAPII) at a single lysine (K1268) by the CRL4CSA ubiquitin ligase. However, how the CRL4CSA ligase is specifically directed toward the K1268 site is unknown. Here, we identify ELOF1 as the missing link that facilitates RNAPII ubiquitylation, a key signal for the assembly of downstream repair factors. This function requires its constitutive interaction with RNAPII close to the K1268 site, revealing ELOF1 as a specificity factor that positions CRL4CSA for optimal RNAPII ubiquitylation. Furthermore, drug-genetic interaction screening reveals an unanticipated compensatory TCR pathway in which ELOF1 together with known factors DOT1L and HIRA protect CSB-deficient cells from collisions between transcription and replication machineries. Our study provides a genetic framework of the transcription stress response and reveals key insights into the molecular mechanism of TCR.