Relationship between the two components of the split promoter of eukaryotic tRNA genes

Plasmids containing eukaryotic tRNA genes are faithfully transcribed in the nucleus of Xenopus laevis oocytes [Cortese, R., Melton, D. A., Tranquilla, T. & Smith, J. D. (1978) Nucleic Acids Res. 5, 4593-4611]. It has been established that two separated regions within the coding sequence of a tRNA gene are essential and sufficient for promotion of transcription [Hofstetter, H., Kressmann, A. & Birnstiel, M. L. (1981) Cell 24, 573-585; Ciliberto, G., Castagnoli, L., Melton, D. A. & Cortese, R. (1982) Proc. Natl. Acad. Sci. USA 79, 1195-1199]. We have constructed a hybrid tRNA gene containing one essential region from tDNALeu and the other from tDNAPro, both from Caenorhabditis elegans. This hybrid gene is efficiently transcribed, thus showing that the essential regions are independent transcriptional signals regardless of the overall regularities of the structure of tRNA genes. We have also constructed mutants of the tRNAPro gene in which the distance between the two essential regions is changed; optimal transcription occurs when this distance is about 40-50 nucleotides.