ADENOVIRUS 12-MEDIATED DOWN-REGULATION OF THE MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) CLASS PROMOTER - IDENTIFICATION OF A NEGATIVE REGULATORY ELEMENT RESPONSIVE TO AD12 E1A

In highly oncogenic adenovirus (Ad) 12-transformed cells, major histoc ompatibility complex (MHC) class I gene expression is down-regulated b y the products of the viral E1A oncogene at the level of initiation of transcription. However, class I gene expression is unaltered or eleva ted in non-oncogenic Ad2- or Ad5-transformed cells. These changes in c lass I expression may permit Ad12-transformed cells to escape host imm une surveillance and elicit tumour formation. Here we show that the 2k b of 5' flanking region of the mouse H-2K(b) class I gene is sufficien t to mediate down-regulation of transcription driven from homologous o r heterologous (HSV thymidine kinase) basal promoter elements in cells expressing Ad12 E1A, but not in Ad2 E1A-expressing cells. Deletion an alysis of the 2kb region showed that sequences from -1.18 to -1.44kb ( relative to the cap site) were a target for Ad12 E1A-mediated transcri ptional dawn-regulation. Deletion of this entire region from the 2kb f lanking sequence of the H-2K(b) gene abolished Ad12 E1A-mediated down- regulation of transcription. Computer analysis of the -1.18 to -1.44kb sequence identified two 6/7bp matches with the AP-1 transcription fac tor consensus sequence anti two matches with the pig MHC class I PD1 r epressor element. Gel retardation analysis using overlapping DNA fragm ents derived from the -1.18 to -1.44kb sequence revealed several DNA:p rotein complexes formed using nuclear extract derived from Ad12-, but not from Ad2- or Ad5-transformed cells. Some of these DNA:protein comp lexes were also present, but at lower levels, in nuclear extracts from untransformed rat cells suggesting the possible involvement of cellul ar factors in the mechanism of down regulation mediated by Ad12 E1A. A binding site for the AP-1 factor failed to compete for protein bindin g to fragments within the -1.18 to -1.44 sequence, while the PD1 site competed for binding only in the -1.15 to -1.23 region. These results indicate that novel factors (as well as a previously identified class I repressor, PD1) may be involved in Ad12 E1A-mediated down-regulation of MHC class I transcription.