Spacer promoters are essential for efficient enhancement of X. laevis ribosomal transcription

The X. laevis ribosomal DNA spacer contains duplicated RNA polymerase I ”spacer promoters“ and an array of repeated 60 81 bp promoter-related sequences. The latter have been shown to enhance transcription from a 40S preribosomal RNA promoter in cis. Here we present evidence that at least one spacer promoter is also necessary for efficient enhancement. Deletion of the spacer promoter sequences in a construct carrying only one such promoter reduces 40S RNA transcription to approximately 10% of wild type. This effect apparently is caused by inactivation of the spacer promoter, since mutants in which 4–177 bp of the spacer promoter and adjacent sequences are deleted are functionally equivalent. Spacer promoters and 60 80 bp arrays therefore probably act together to enhance 40S pre-RNA transcription in X. laevis.