A prokaryotic tRNATyr gene, inactive in Xenopus laevis oocytes, is activated by recombination with an eukaryotic tRNAPro gene.

Eukaryotic tDNA promoters are composed of two essential regions contained within the coding sequence (Box A and Box B). Due to the highly conserved structure of prokaryotic and eukaryotic tRNA, most prokaryotic tRNA genes are expected to be active templates in eukaryotic transcriptional systems. In this paper we show that Escherichia coli tDNATyr is not transcribed in the nucleus of Xenopus laevis oocytes. By in vitro construction of hybrid molecules between inactive prokaryotic tDNATyr from E. coli, and active eukaryotic tDNAPro from Caenorhabditis elegans, we show that tDNATyr can be made into an active gene if its first third, including the Box A region, is replaced by that of the eukaryotic tDNA . These results suggest that an improper Box A sequence is responsible for the inactivity of the E. coli tRNATyr gene, and argue against the role of secondary and tertiary DNA conformations in RNA polymerase III transcription.