Since the recognition of Archaea as a separate domain of life, interest in the fascinating organisms in this unique evolutionary lineage has continued to grow. Archaea are found in a wide range of extreme environments, including those with high temperature (hyperthermophiles), high osmotic pressure (saturated salt conditions; halophiles) and extreme pH (acidophiles and alkalophiles), often in combination with anaerobic conditions. In addition, non-extremophilic archaea are globally distributed and abundant in both marine and terrestrial environments.
Archaea have several distinguishing features, including unique membrane constituents, whereas other characteristics are shared either with bacteria or with eukaryotes. For example, despite the fact that archaeal cells lack a nucleus, the majority of archaeal proteins involved in replication, transcription and translation have homology to eukaryotic counterparts, whereas the corresponding bacterial proteins are significantly more distantly related, if at all. The crenarchaeotal cell division and cytoskeletal complexes also comprise proteins with eukaryotic homologues. Bacterial-like features include the presence of single circular chromosomes (although exceptions exist), the organization of a large fraction of genes into operons, the euryarchaeotal division machinery, and the presence of a wide range of bacterial-type transcription factors.
Growth and proliferation of all cell types requires intricate regulation and coordination of chromosome replication, genome segregation, cell division and the systems that determine cell shape. Recent findings have provided insight into the cell cycle of archaea, including the multiple-origin mode of DNA replication, the initial characterization of a genome segregation machinery and the discovery of a novel cell division system. The first archaeal cytoskeletal protein, crenactin, was also recently described and shown to function in cell shape determination.
This review outlines the current understanding of the archaeal cell cycle and cytoskeleton, with an emphasis on species in the genus Sulfolobus, and consider the major outstanding questions in the field.