Sequence analysis and expression in cultured lymphocytes of the human FOSB gene (G0S3). 
G0S3 is a member of a set of putative G0/G1 switch regulatory genes (G0S genes) 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 murine FOSB gene, which encodes a component of the AP1 transcriptional regulator. Comparison of cDNA and genomic sequences reveals a 4-exon structure characteristic of the FOS family of genes. Freshly isolated cells show high levels of FOSB/G0S3 and FOS/G0S7 mRNAs, 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 of FOS mRNA reach a maximum 20 min after the addition of lectin and decline to control levels over the next 3 hr. Levels of FOSB mRNA reach a maximum 40 min after the addition of lectin and decline to control levels over the next 6 hr. In freshly isolated cells, both FOS and FOSB mRNAs increase dramatically in response to the protein synthesis inhibitor cycloheximide. In preincubated cells, the cycloheximide response is decreased, especially in the case of FOSB. These differences in expression of FOS and FOSB suggest 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 the FOSB gene suggest that the potential may help locate genes in uncharted DNA sequences.