We have detected a DNAseI hypersensitive site in the ribosomal DNA spacer of 
Xenopus laevis and Xenopus borealis. The site is present in blood and embryonic 
nuclei of each species. In interspecies hybrids, however, the site is absent in 
unexpressed borealis rDNA, but is present normally in expressed laevis rDNA. 
Hypersensitive sites are located well upstream (over lkb) of the pre-ribosomal 
RNA promoter. Sequencing of the hypersensitive region in borealis rDNA, however, 
shows extensive homology with the promoter sequence, and with the hypersensitive 
region in X. laevis. Of two promoter-like duplications in each spacer, only the 
most upstream copy is associated with hypersensitivity to DNAaseI. Unlike 
DNAaseI, Endo R. MspI digests the rDNA of laevis blood nuclei at a domain 
extending downstream from the hypersensitive site to near the 40S promoter. 
Since the organisation of conserved sequence elements within this "proximal 
domain" is similar in three Xenopus species whose spacers have otherwise evolved 
rapidly, we conclude that this domain plays an important role in rDNA function.