c- mos Upstream sequence exhibits species-specific enhancer activity and binds murine-specific nuclear proteins

Expression of the c- mos proto-oncogene is detected only in mouse and rat testis and ovaries. Its transcriptionally silent character suggested the involvement of sequences that cause repression of the c- mos expression. Evidence for such repressor sequences has been reported. Here we investigated the possibility that a cis-acting enhancer element(s) is responsible for the cell type-specific c- mos(rat) expression. We localized an enhancer close to (300 bases) the repressor sequences, which acts on heterologous promoters in an orientation-independent manner over large distances. Surprisingly, it shows species preference. Sequence analysis of the enhancer fragment indicated the presence of four blocks of nucleotides homologous to the simian virus 40 enhancer core sequence. Different nuclear proteins, some of which are expressed in murine but not in primate cells, bind to the enhancer DNA. They protect three closely associated areas from DNase I digestion, two of which are homologous to the simian virus 40 enhancer core sequence. The third protected sequence is located downstream from the two enhancer core blocks.