An evolutionarily conserved target motif for immunoglobulin class-switch recombination

Immunoglobulin H class-switch recombination (CSR) occurs between switch regions and requires transcription and activation-induced cytidine deaminase (AID). Transcription through mammalian switch regions, because of their GC-rich composition, generates stable R-loops, which provide single-stranded DNA substrates for AID. However, we show here that the Xenopus laevis switch region Sμ, which is rich in AT and not prone to form R-loops, can functionally replace a mouse switch region to mediate CSR in vivo. X. laevis Sμ–mediated CSR occurred mostly in a region of AGCT repeats targeted by the AID–replication protein A complex when transcribed in vitro. We propose that AGCT is a primordial CSR motif that targets AID through a non-R-loop mechanism involving an AID–replication protein A complex.