Transcriptional control of MHC class I and beta 2-microglobulin genes in vivo.

The expression of class I and beta 2-microglobulin (beta 2-m) genes, which encode the H and L chains of the H-2 histocompatibility Ag, respectively, is complex both in the adult mouse and during development. Although they are ubiquitously expressed in the adult, the mRNA levels of these genes are variable from one organ to another, being high in liver, lung, and lymphoid organs and low in brain and testis. During development, both class I and beta 2-m mRNA are poorly expressed. To determine the molecular mechanism, either transcriptional or post-transcriptional, controlling class I and beta 2-m mRNA levels, we have compared their transcriptional activities by performing run-on experiments with nuclei extracted from several embryonic and adult organs. We show that most of the differences observed in H-2 and beta 2-m mRNA steady state levels are the reflection of their different transcriptional activities. These results demonstrate that MHC class I and beta 2-m gene expression in adult organs, as well as during development, is mainly controlled at the transcriptional level.