Specificity and Function of Archaeal DNA Replication Initiator Proteins

Summary

Chromosomes with multiple DNA replication origins are a hallmark of Eukaryotes and some Archaea. All eukaryal nuclear replication origins are defined by the origin recognition complex (ORC) that recruits the replicative helicase MCM(2-7) via Cdc6 and Cdt1. We find that the three origins in the single chromosome of the archaeon Sulfolobus islandicus are specified by distinct initiation factors. While two origins are dependent on archaeal homologs of eukaryal Orc1 and Cdc6, the third origin is instead reliant on an archaeal Cdt1 homolog. We exploit the nonessential nature of the orc1-1 gene to investigate the role of ATP binding and hydrolysis in initiator function in vivo and in vitro. We find that the ATP-bound form of Orc1-1 is proficient for replication and implicates hydrolysis of ATP in downregulation of origin activity. Finally, we reveal that ATP and DNA binding by Orc1-1 remodels the protein’s structure rather than that of the DNA template.

Abstract

Graphical Abstract

Highlights

► The S. islandicus chromosome has three origins, each with its own initiator ► Two origins are Orc dependent, and one requires a Cdt1 homolog ► The ATP-bound form of Orc1 is proficient at MCM loading ► ATP binding remodels the protein structure, not that of the DNA template

Abstract

Archaea of the genus Sulfolobus use three replication origins per chromosome. She, Bell, and colleagues show that the three origins in S. islandicus have distinct initiator proteins, making this chromosome a mosaic of replicons. The nonessential nature of the Orc1/Cdc6 genes permits combined in vitro and in vivo analyses of their function. These findings reveal that ATP binding, not hydrolysis, is required for Orc1 function and that ATP exerts its effect by remodeling the initiator protein, not the origin DNA.