ROLE OF THE Q10 CLASS-I REGULATORY ELEMENT REGION-1 IN CONTROLLING TISSUE-SPECIFIC EXPRESSION IN-VIVO

The MHC class I regulatory element (CRE) region 1 has been previously described as a positive cis-acting regulatory element essential for cl ass I gene expression. We have generated transgenic mice (CBA x C57BL/ 6) with the MHC class I gene H-2D(d) driven by two different 400-bp pr omoter regions of Q10, a nonpolymorphic MHC class I gene expressed in the liver, kidney, and fetal yolk sac. One transgene contained the wil d-type Q10 promoter (Q10WT/D(d)). The second construct (Q10M3/D(d)) ha d 2 bp substitutions introduced in region 1 of the CRE that reconstitu ted the CRE inverted repeat present in classical class I genes. Mice c ontaining the wild-type Q10/D(d) gene expressed membrane-bound H-2D(d) molecules in a tissue-restricted expression pattern similar to that o bserved for endogenous Q10. In mice containing the mutant construct (Q 10M3/D(d)), H-2D(d) was also expressed in the thymus, a tissue not nor mally associated with Q10 expression but, surprisingly, the D(d) was n ot expressed in other lymphoid tissues. Furthermore, thymic expression was greatest on double positive (CD4+ CD8+) thymocytes. Thymic D(d) e xpression was correlated with the presence of what appears to be a pre viously unidentified transcription factor in thymocytes that is capabl e of interacting with the CRE-inverted repeat. These results show that the mutations in region 1 altered the tissue-specific regulation of t he Ql 0 promoter in vivo, although an intact inverted repeat did not r estore the ubiquitous pattern of expression characteristic of classica l class I genes. Thus, these results indicate that elements in additio n to CRE region 1 in the Q10 promoter region serve to limit ubiquitous tissue expression.