RECOMBINATION POTENTIAL OF THE HUMAN DIR ELEMENTS

The human DIR genes are putative D-H elements that are GC rich and are found between DN and DM D-H gene segments. The DIR genes each have co nsensus recombination signal sequences (RSS) that could conceivably ge nerate DIR coding regions of about 126 nucleotides. These RSS should a llow for D-H-D-H rearrangements that do not violate the 12/23 recombin ation rule. Several Ig CDR3 sequences have been assigned to DIR usage; however, there are frequent gaps and mismatches associated with these assignments. In some instances these CDR3 sequences might be better e xplained by GC rich N segment addition. This report analyzes the recom bination potential of the human DIR elements by PCR. Though D-H-J(H) r earrangement of the D-H genes flanking the DIR regions (DM and DN) are easily demonstrated, very little evidence of DIR-J(H) rearrangement c ould be documented. Furthermore, in amplifications that should concurr ently detect both D-H-DIR rearrangements (which maintain the 12/23 rec ombination rule) and D-H-D-H rearrangements (which violate the 12/23 r ecombination rule), D-H-D-H rearrangements predominate. We conclude th at the DIR-associated RSS participate minimally in both D-H-D-H and D- H-J(H) rearrangement. In addition, we describe several conventional D- H-D-H rearrangement intermediates, demonstrating unequivocally that th is phenomena occurs during human Ig rearrangement.