Cellular transcripts encoded at a locus which permits retrovirus expression in mouse embryonic cells

Three independent recombinant retroviruses have been activated on insertion into the F2 locus of mouse F9 embryonal carcinoma cells. Each provirus has integrated downstream from the cellular F2 promoter, which is active in transient transfection assays using a chloramphenicol acetyltransferase reporter enzyme. The F2 promoter drives expression of a series of related transcripts in F9 and 3T3 cells, and a single 450-nt transcript in mouse tissues. F2 homologous sequences have been detected in the genomes of all mammalian species tested, and the 450-nucleotide (nt) F2 transcript is expressed in rat and human cells. Three pairs of differently sized F2 cDNA clones have been isolated and analyzed. The largest clones possess two 199-nt 98.5% identical repeats, one of which is present in the smaller clones, as well as the major 450-nt transcript. Activated proviral integration sites map to introns of the largest F2 cDNA clone. While none of the F2 cDNA contains a long open reading frame or homology to databank sequences, evidence suggests that the F2 locus encodes a constitutive function required at high levels, or represents an expressed but nonfunctional, single-copy element, conserved among mammals.