Spacer sequences regulate transcription of ribosomal gene plasmids injected into Xenopus embryos

We have injected plasmids containing a repeating unit (spacer plus gene) of Xenopus laevis ribosomal DNA into fertilized eggs. Transcription on these plasmids begins at the same time as transcription on the endogenous ribosomal genes (late blastula stage). Previous work defined the ribosomal gene promoter as the region from -140 to +6 around the site of transcription initiation (Reeder et al., 1982; Moss, 1982; Sollner-Webb et al., submitted). We show here that the level of transcription of the injected ribosomal genes is strongly affected by spacer sequences far upstream of the promoter. Deletion of spacer sequences over 1150 bp from the initiation site reduces the transcription signal from injected plasmids by a factor of 5-10. We propose that the upstream spacer sequences act to influence the frequency of promoter activation.