Molecular cloning and characterization of the murine Staf cDNA encoding a transcription activating factor for the selenocysteine tRNA gene in mouse mammary gland

We have isolated and characterized a cDNA encoding a transcription activating factor for the mouse selenocysteine tRNA (tRNA(sec)) gene from mouse mammary gland. The full-length cDNA, designated m-Staf, has a 1878-base pair open reading frame encoding 626 amino acids. The predicted amino acid sequence of m-Staf is highly homologous to that of Staf, another selenocysteine tRNA gene transcription activating factor of Xenopus laevis. Like Staf, m-Staf contains seven tandemly repeated zinc fingers and four repeated motifs. Gel shift assays indicated that the recombinant m-Staf specifically bound to the activator element region in the mouse tRNA(sec) gene. Transient co-transfection experiments in Drosophila Schneider cells, which lack endogenous Staf-like binding activity, showed that m-Staf increased the mouse tRNA(sec) gene transcription about 15-fold, whereas it stimulated Pol II-dependent thymidine kinase promoter only 2-fold. Northern blot analysis detected the presence of a 3.4-kilobase pair m-Staf transcript, which was widely but differentially expressed in various murine tissues. The binding activity of m-Staf in mouse mammary gland was undetectable during virgin and postlactating periods but increased markedly in parallel with the increase of tRNA(sec) transcript during the periods of pregnancy and lactation, when the gland undergoes growth and development. These results indicate that m-Staf is a transcriptional activator of the mouse tRNA(sec) gene and that its binding activity in the mammary gland undergoes developmental alterations.