A specific replication origin in the chromosomal rDNA of Lytechinus variegatus.

Abstract

Specific replication origins have been well characterized in prokaryotes1,2, and have been identified in eukaryotic extra-chromosomal DNAs3–8. The best candidate, until now, for a specific eukaryotic chromosomal replication origin has been found in yeast9, but the general existence of such origins is challenged10 by the finding that SV40 and polyoma DNA fragments lacking the viral replication origin can replicate after injection into Xenopus eggs. It has thus been suggested that the origins found on extrachromosomal DNAs exist solely to circumvent the cells' strict requirement for chromosomal DNA to replicate once per cell division. Clearly the disparate views can only be reconciled after the replication of specific chromosomal genes has been studied. To this end, we have studied the replication of the ribosomal genes (rDNA) of the sea urchin Lytechinus variegatus. The visualization of replicating rDNA, isolated from rapidly dividing sea urchin gastrula cells, after restriction endonuclease digestion demonstrates that the initiation of replication in these chromosomal genes is sequence-specific, and is most probably confined to a region within the non-transcribed spacer.