Eukaryotic chromosomes possess multiple origins of replication, whereas 
bacterial chromosomes are replicated from a single origin. The archaeon 
Pyrococcus abyssi also appears to have a single origin, suggesting a common rule 
for prokaryotes. However, in the current work, we describe the identification of 
two active origins of replication in the single chromosome of the 
hyperthermophilic archaeon Sulfolobus solfataricus. Further, we identify 
conserved sequence motifs within the origins that are recognized by a family of 
three Sulfolobus proteins that are homologous to the eukaryotic initiator 
proteins Orc1 and Cdc6. We demonstrate that the two origins are recognized by 
distinct subsets of these Orc1/Cdc6 homologs. These data, in conjunction with an 
analysis of the levels of the three Orc1/Cdc6 proteins in different growth 
phases and cell cycle stages, lead us to propose a model for the roles for these 
proteins in modulating origin activity.