Figure 1.

Switch recombination alters the immunoglobulin heavy chain locus. (A) The murine heavy chain locus is diagrammed in the top line, which shows the rearranged variable region [V(D)J], the intron enhancer (Eµ), the switch (S) and constant (C) regions. Recombination from µ (IgM) to γ1 (IgG1) expression will result in joining of the Sµ and Sγ1 switch regions at sites that are heterogeneous in sequence and in position, as diagrammed below. (B) The LR1 site in the core Eµ enhancer. The region of the murine heavy chain intron enhancer (Eµ) which contains the binding sites for LR1, E4 and OCTA (underlined) is shown. (C) CAT expression driven from Eµ reporter constructs carrying wild-type or mutant LR1 sites. The graph shows levels of transcription activation as a percentage of total activation by pIgE-P-CAT. Mock, no DNA control.