ABSTRACTG0S3is a member of a set of putative G0/G1 switch regulatory genes (G0Sgenes) selected by screening cDNA libraries prepared from human blood mononuclear cells cultured for 2 hr with lectin and cycloheximide. The sequence shows high homology with the murineFOSBgene, which encodes a component of the API transcriptional regulator. Comparison of cDNA and genomic sequences reveals a 4-exon structure characteristic of theFOSfamily of genes. Freshly isolated cells show high levels ofFOSB/G0S3andFOS/G0S7mRNAs, which decline rapidly during incubation in culture medium. The kinetics of expression suggest that the high initial levels are caused by the isolation procedure, and do not reflect constitutive expression. In cells preincubated for a day, levels ofFOSmRNA reach a maximum 20 min after the addition of lectin and decline to control levels over the next 3 hr. Levels ofFOSBmRNA reach a maximum 40 min after the addition of lectin and decline to control levels over the next 6 hr. In freshly isolated cells, bothFOSandFOSBmRNAs increase dramatically in response to the protein synthesis inhibitor cycloheximide. In preincubated cells, the cycloheximide response is decreased, especially in the case ofFOSB. These differences in expression ofFOSandFOSBsuggest different roles and regulation. Regions of low base order-dependent stem–loop potential in the region of the gene are defined. These indicate where base order has been adapted for purposes other than stem–loop stability (e.g., encoding proteins or gene regulation). Regions of low potential in a 68.5-kb genomic segment containing theFOSBgene suggest that the potential may help locate genes in uncharted DNA sequen
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