Products and implied mechanism of H chain switch recombination.

The Ig H chain switch is a DNA recombination event. The recombination occurs between two or more switch regions, areas of tandem sequence duplication that lie upstream of the corresponding H chain C region genes. We have determined the DNA sequence at four recombination sites in three molecularly cloned, rearranged switch regions. All eight donor and recipient recombination sites are at the common pentamers GGGGT, GAGCT, and GGTGG. One of the switch recombination events is an inversion of S gamma 3 sequences. Another of the recombinational events is an internal S gamma 1 deletion, which may be switch enzyme mediated. These results, together with other switch recombination site sequences, suggest that switch recombination is mediated by cutting enzymes with modest specificity and religation enzymes with no specificity.