InVivoFunction ofRegulatory DNA Sequence Elements ofa MajorHistocompatibili ty ComplexClassIGene

complex class Igenesareexpressed innearly allsomatic tissues, although their level ofexpression varies. Byanalysis ofa setofpromoter deletion mutants introduced intotransgenic mice, a complex regulatory element, consisting ofoverlapping enhancer andsilencer activities, isdemonstrated to function as a tissue-specific regulator ofclass Iexpression. Theenhancer activity predominates inlymphoid tissues butnotinnonlymphoid tissues. Incontrast tothetissue-specific function ofthecomplex regulatory element, a second novel silencer element isshowntofunction inbothlymphoid andnonlymphoid tissues. The complement ofDNA-binding factors indifferent celllines isshowntocorrelate withthelevels ofclass I expression.