Recombinant plasmids that carried genes coding for 5S ribosomal RNA of the newt, Notophthalmus viridescens, were transcribed in vitro with extracts of Xenopus laevis oocyte nuclei. Plasmids containing multiple repeats of the 5S gene and spacer directed accurate transcription of 5S RNA (120 bases). Individual repeat units were recloned by inserting Sau 3A restriction fragments into the Bam HI site of plasmid pBR322. Because each repeat was cut by the enzyme within the coding region, the inserts had incomplete coding regions at their ends and spacer sequences in the middle. The DNA of these subclones directed synthesis of a 5S-size RNA that contained both plasmid and 5S RNA sequences. Transcription initiated in the vector, proceeded through the gene segment coding for nucleotides 41-120, and terminated at the end of the gene. The initiation of in vitro transcription required neither the original 5' flanking sequences of the spacer nor the first third of the gene. We conclude that intragenic DNA sequences control the initiation of transcription. Other subclones that include pseudogenes gave rise to some transcripts 156 nucleotides long. These long transcripts represented continuation of transcription through the 36-base-pair pseudogene that is located immediately downstream from the 5S gene. However, most transcripts of these subclones terminated at the end of the normal gene before the beginning of the pseudogene. It is probable that a run of four or more Ts serves as part of the termination signal.
In vitro transcription of cloned 5S RNA genes of the newt Notophthalmus.