Spacer regulation of Xenopus ribosomal gene transcription: competition in oocytes.

Xenopus laevis ribosomal gene plasmids bearing different length spacers were injected into oocyte nuclei in competition with each other. The spacer has two basic effects on transcription from the gene promoter. First, if the competing pair have unequal spacer lengths, the gene promoter attached to the longer spacer is always dominant in transcription (the competition effect). Second, as the total amount of spacer in the reaction increases, the total amount of transcription decreases (the sink effect). Both the competition and the sink effect are attributed to sequence elements that are 60 or 81 bp long, which are present in multiple copies in the spacer and are related in sequence to part of the gene promoter. The 60/81 bp elements confer competitive dominance in either orientation. A model which explains both the competition and the sink effects is discussed in which the 60/81 bp elements are attraction sites for a factor(s) which is needed to activate the gene promoter.