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.

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