The DSIF subunits Spt4 and Spt5 have several unexpected, distinct roles in regulating immunoglobulin class switch recombination

Class-switch recombination (CSR), induced by activation-induced cytidine deaminase, can be divided into two phases: DNA cleavage of the switch (S) regions, and the joining of the cleaved ends of the different S regions. Here, we show that the DSIF complex (Spt4 and Spt5), a transcription elongation factor, is required for CSR in CH12F3-2A cells, and Spt4 and Spt5, carry out independent functions in CSR. Depleting Spt5 reduced the H3K4me3 levels and DNA cleavage at the Sα region, whereas Spt4 knockdown did not perturb the H3K4me3 status or S region cleavage. Thus, we conclude that Spt5 plays a role similar to the histone chaperone FACT by regulating histone modification and DNA cleavage in CSR. Assay systems using synthetic repair substrates revealed that both Spt4 and Spt5 are required for non-homologous end joining and homologous recombination. Taken together, Spt4 and Spt5 are involved in multiple CSR steps, and can function independently. Spt4 or Spt5 were knocked down in CH12F3-2A cells in the presence of CD40L, IL4, TGFb (CIT) stimulation. RNA from each samples were extracted and relative difference in transcript level were compared with control RNAi-treated, CIT-stimulated samples.