A complex array of sequences enhances ribosomal transcription in Xenopus laevis

The ribosomal DNA spacer in Xenopus laevis was shown in previous studies to be involved in regulating the expression of the ribosomal genes. Here transcription enhancement by this spacer has been studied in some detail, to fully identify the sequences involved and to determine their relative importance in this phenomenon. It is shown that the 60/81 base-pair (bp) repeats, which were reported to be enhancer elements, act as part of a mode of enhancement whose effect is amplified by the spacer promoters or Bam islands. The "Bam super repeat", a combination of spacer promoter and 60/81 bp elements, is the major enhancer unit. Within a Bam super repeat, a near linear correlation between the number of 60/81 bp elements and enhancer activity is observed. Thus, there is no significant co-operativity in the binding of transcription factors to an array of these elements. Multiple Bam super repeats do not act additively and may actually interfere with each others action. Surprisingly this effect is observed both in the presence and absence of active spacer promoters. Sequences between the 3' end of the 28 S coding region and the first spacer promoter may also be involved in enhancement but only in a very minor fashion. In confirmation of recent studies, the presence of the unique ribosomal termination sequence, 213 bp upstream from the pre-rRNA initiation site, is essential for efficient promotion, as deletion of this sequence virtually abolishes pre-rRNA- transcription. These data are discussed in terms of the possible mechanisms of transcription enhancement.