Regulation of Tcrb recombination ordering by c-Fos-dependent RAG deposition

Antigen receptor variable-(diversity)-joining (V(D)J)recombination at the locus encoding the T cell antigen receptor-beta (Tcrb)is ordered, with D-beta-to-J(beta) assembly preceding V-beta-to-DJ(beta) joining. The molecular mechanism underlying this 'preferred' order of rearrangement remains unclear. Here we show that the D-beta 23-base pair recombination signal sequence (D-beta 23-RSS)contains a specific AP-1 transcription factor-binding site bound by AP-1 and its component c-Fos expressed at a specific stage. Cell-based recombination assays suggested that c-Fos interacted directly with the RAG recombinase and enhanced its deposition to Db 23-RSSs, thus conferring the priority of DJ(beta) recombination. Loss of c-Fos decreased Tcrb recombination efficiency and disrupted recombination ordering in vivo. Our results show an unexpected function for c-Fos as a direct regulator of Tcrb recombination, rather than its usual function as a transcription regulator, and provide new insight into the mechanisms of recombination ordering.