Biased reading frames of pre‐existing DH‐‐JH coding joints and preferential nucleotide insertions at VH‐‐DJH signal joints of excision products of immunoglobulin heavy chain gene rearrangements.

During B cell differentiation immunoglobulin (Ig) DH segments join to JH segments, followed by joining of VH to DJH. Although circular excision products of DH‐‐JH rearrangements have been characterized, excision products of VH to DJH joining have never been isolated. We selectively denatured chromosomal DNA of mouse splenocytes and enriched circular DNA spanning the long distance between VH and DH. Subsequent PCR amplifications allowed the identification of signal joints of VH to DJH. Sequence analysis indicated that preexisting DH‐‐JH coding joints of excision products showed a strong bias for reading frame 1, and the absence of reading frame 2, which would allow the expression of a truncated mu chain called D mu protein. When comparing the joints of the VH‐‐DJH and DH‐‐JH rearrangements we observed N‐nucleotide insertions to be abundant at the VH‐‐DH signal joint, but very sparse at the DH‐‐JH signal joint, while the coding joints of both contained abundant N‐insertions. These differences in N region insertions at the signal joints suggest a differential control of the D‐‐J and V‐‐DJ rearrangements.