The gene family encodes 21K proteins that reside on the inner face of the plasma membrane and bind GTP and GDP with an equally high affinity. Cotransfection of NIH 3T3 cells with a mammalian expression vector containing a viral Harvey- (v-Ha-) cDNA, together with a plasmid (pCMVCAT) carrying the immediate early (IE) enhancer of the murine cytomegalovirus (MCMV) linked to the chloramphenicol acetyltransferase (CAT) reporter gene strongly stimulated CAT activity. Basal levels of pCMVCAT expression as well as trans-activation by v- plasmid were both inhibited by cotransfection of an expression vector containing the dominant inhibitory mutant gene Ha- Asn-17. This indicates that the p21 protein is responsible for these activities. High pCMVCAT activation was also observed in cell lines carrying stably transfected oncogenes, activated by point mutation or amplification. To define the cis-acting DNA elements in the MCMV IE enhancer responsible for this trans-activation by p21 protein, we constructed several plasmids containing the CAT gene under control of MCMV IE enhancers that were deleted in different regions. The CAT assays demonstrated that several sequences were responsive to p21 protein. These sequences are scattered throughout the IE enhancer, upstream of the transcription start site, and contain responsive elements that are homologous to the binding sites for cellular transcription factors such as NFκB, AP1, ATF and SP1. Activation of the p21 protein may thus be one of the signals that regulate IE genes transcription during MCMV infection.


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