Maximization of selenocysteine tRNA and U6 small nuclear RNA transcriptional activation achieved by flexible utilization of a Staf zinc finger

Transcriptional activators Staf and Oct-1 play critical roles in the activa tion of small nuclear RNA (snRNA) and snRNA-type gene transcription. Recent ly, we established that Staf binding to the human U6 snRNA (hU6) and Xenopu s selenocysteine tRNA (xtRNA(Sec)) genes requires different sets of the sev en C2-H2 zinc fingers. In this work, using a combination of oocyte microinj ection, electrophoretic mobility shift assays, and missing nucleoside exper iments with wild-type and mutant promoters, me demonstrate that the hU6 gen e requires zinc fingers 2-7 for Staf binding and Oct-1 for maximal transcri ptional activity. In contrast, the xtRNA(Sec) gene needs the binding of the seven Staf zinc fingers, but not Oct-1, for optimal transcriptional capaci ty. Mutation in the binding site for Staf zinc finger 1 in the tRNA(Sec) pr omoter reduced both Staf binding and transcriptional activity. Conversely, introduction of a zinc finger 1 binding site in the hU6 promoter increased Staf binding but interfered with the simultaneous Staf and Oct-1 binding, t hus reducing transcriptional activity. Collectively, these results show tha t the differential utilization of Staf zinc finger 1 represents a new, crit ical determinant of the transcriptional activation mechanism for the Xenopu s tRNA(Sec) and human U6 snRNA genes.