MULTIPLE SEQUENCES FROM DOWNSTREAM OF THE J(KAPPA) CLUSTER CAN COMBINE TO RECRUIT SOMATIC HYPERMUTATION TO A HETEROLOGOUS, UPSTREAM MUTATION DOMAIN

Recruitment of somatic hypermutation to the Ig chi locus has previousl y been shown to depend on the enhancer elements, Ei/MAR and E3'. Here we show that these elements are not sufficient to confer mutability. H owever, hypermutation is effectively targeted to a chimeric beta-globi n/Ig chi transgene whose 5' end is composed of the human beta-globin g ene (promoter and first two exons) and whose 3' end consists of select ed sequences derived from downstream of the J(chi) cluster (Ei/MAR, C- chi + flank and E3'). Thus, multiple downstream Ig chi sequences (all derived from 3' of the J(chi) cluster) can combine to recruit mutation to a heterologous mutation domain. The location of this hypermutation domain is defined by the position of the transcription start site and this applies even if the Ig chi Ei/MAR is positioned upstream of the promoter. Hotspots within the mutation domain are, however, defined by local DNA sequence as evidenced by a new hotspot being created within the beta-globin domain by a mutation within the transgene. We propose that multiple, moveable Ig chi sequences (that are normally located d ownstream of the transcription start site) cooperate to bring a hyperm utation priming factor to the transcription initiation complex; a muta tion domain is thereby created downstream of the promoter but the loca l sequence defines the detailed pattern of mutation within that domain .