Germ line transcription in mice bearing neor gene downstream of Ic3 exon in the Ig heavy chain locus

Abstract

Class switch recombination (CSR) is preceded by germ line transcription that initiates from promoters upstream of switch (S) sequences and terminates downstream of associated constant genes. Previous work showed that germ line transcripts and their processing are required for CSR and that germ line transcription is regulated in a major part by a regulatory region located downstream of the Ig heavy chain locus. This long-range, polarized effect can be disturbed by inserting an expressed neomycine resistance (neor) gene. To contribute to a better understanding of the mechanism of such a long-distance regulation, we generated knock-in mice in which a neor gene was inserted downstream of Ic3 exon leaving intact all the necessary elements for germ line transcription and splicing. We show that the expressed neor gene interferes with transcription initiation from Ic3, and that it impairs but does not block S recombination to Cc3. Moreover, we show for the first time that the neor gene provides through chimeric neor -Cc3 transcripts the necessary elements for splicing of germ line transcripts by activating two novel cryptic splice sites, one in the coding region of the intronless neor gene and the other in the Ic3-Cc3 intron.