BACKGROUND: Species of the crenarchaeon Sulfolobus harbour three replication 
origins in their single circular chromosome that are synchronously initiated 
during replication.
RESULTS: We demonstrate that global gene expression in two Sulfolobus species is 
highly biased, such that early replicating genome regions are more highly 
expressed at all three origins. The bias by far exceeds what would be 
anticipated by gene dosage effects alone. In addition, early replicating regions 
are denser in archaeal core genes (enriched in essential functions), display 
lower intergenic distances, and are devoid of mobile genetic elements.
CONCLUSION: The strong replication-biased structuring of the Sulfolobus 
chromosome implies that the multiple replication origins serve purposes other 
than simply shortening the time required for replication. The higher-level 
chromosomal organisation could be of importance for minimizing the impact of DNA 
damage, and may also be linked to transcriptional regulation.