DELETIONS OF IMMUNOGLOBULIN C-KAPPA REGION CHARACTERIZED BY THE CIRCULAR EXCISION PRODUCTS IN MOUSE SPLENOCYTES

We have identified circular DNAs containing the kappa-light chain constant region (C-kappa), as well as the excision products of V-kappa-J-kappa and V-lambda-J-lambda joining in adult mouse splenocytes. Analysis of C-kappa- positive circular DNA clones revealed two recombination sites (intron recombining sequence [IRS]1 and -2) within the germline J-kappa-C-kappa-intron region and the recombining sequence (RS) located downstream of the C-kappa-exon. While RS contains a conserved heptamer and nonamer separated by a 23-bp spacer on the 5' side, IRS1 sequence is an isolated heptamer without an obvious nonamer, and IRS2 contains a variant heptamer, CACAAAA. Since IRS1 and IRS2 recombined with both RS (23-bp spacer signal) and V-kappa (12-bp spacer signal) with significant frequency, intron recombination sites seem to have dual recombination signals. These findings provide direct evidence that C-kappa-deletion preceding lambda-gene rearrangement can occur by looping out and excision. Increased accessibility of inefficient recombinational loci within the intron may enable recombinase to accept wide signal sequence variation.