PROMOTER, ENHANCER AND SILENCER ELEMENTS REGULATE REARRANGEMENT OF ANIMMUNOGLOBULIN TRANSGENE

The chicken Ig lambda light chain locus is composed of a single V gene closely linked (1.8 kb) to a single J-C unit in its natural configura tion. In mice transgenic for this locus, the transgene becomes rearran ged in B cells and to a much lesser extent in T cells. Modifications w ere introduced in the transgene in order to characterize elements whic h target the recombinase to the Ig loci. In the absence of either the promoter or the enhancer located 3' of Clambda, rearrangement of the t ransgene is reduced 20- to 100-fold. Moreover, rearrangement is increa sed 5-fold when the DNA segment between Vlambda and Jlambda ('Uo segme nt'), which is deleted during the joining process, is replaced by a ne utral DNA segment of equal length. The Uo segment behaved as a strong transcriptional silencer when tested in a CAT assay in vitro. Control transgenic mice harbouring only the two 3 bp mutations that introduced restriction sites at both ends of the Uo segment to allow for its rep lacement were also analysed. Rearrangement was reduced 10- to 100-fold in B cells from such transgenic lines. A model is proposed whereby th e sites of these two mutations would function by counteracting transie ntly the repressing effect of the silencer, thus giving access of the chicken light chain locus to the recombinase.