Abstract
The function of the X. laevis ribosomal spacer was studied in vivo and by microinjection of in vitro mutants into Xenopus oocytes. The spacer directs specific RNA transcripts which most probably terminate upstream of the ribosomal genes and it is able to modulate transcription of these genes. The data lead to a model in which the ribosomal spacer is a loading site for RNA polymerase I and spacer transcription is the driving force by which polymerase is delivered to the ribosomal gene promoter.