SEQUENCES AFFECTING THE V(D)J RECOMBINATIONAL ACTIVITY OF THE IGH INTRONIC ENHANCER IN A TRANSGENIC SUBSTRATE

The immunoglobulin heavy chain intronic transcriptional enhancer (Emu) is part of a complex cis-regulatory DNA region which has notably been shown to modulate V(D)J rearrangements of associated variable gene se gments. We have used recombination substrates comprised of the Emu enh ancer together with various lengths of additional downstream mu sequen ces to assess the individual contribution of those sequences to the V( D)J recombinational regulatory activity. Surprisingly, in the absence of large amounts of mu sequences, substrate rearrangements were not de tected in Southern blot analyses of the lymphoid tissues from independ ent transgenic mice, but were readily detectable following transfectio n into cultured pre-B cells. A short mu segment which includes matrix association regions (MARs) was not sufficient to restore high levels o f rearrangements within the reporter transgenes. However, additional e xperiments demonstrated that the mu sequences are dispensable for V(D) J recombination in transgenic thymuses, implying a suppressive effect exerted by the vector sequences left in the transgenic insert, when th ey are attached near the Emu regulatory region. This suppression of V( D)J recombination, which correlates with an hypermethylation of the tr ansgenes, is discussed in view of previously reported transgenic and g ene targeting experiments.