Archaeal characteristics (from Trends in Microbiology 8, 278-283)


Cellular properties:
Prokaryotic; no membrane-surrounded nucleus or eukaryal-type organelles. Membrane lipids ether linked to sn-glycerol-1-phosphate; sometimes tetraethers form a monolayer membrane. Cell wall composition variable, bacterial-type murein absent.

Chromosomes and replication:
Eukaryal-type histones and nucleosomes present in euryarchaea, but not in crenarchaea. Usually single (exceptions exist) circular chromosome. Single replication origin, possibly multiple origins in some species. Present candidate for replication initiator is eukaryal-type Cdc6 and/or other proteins. DNA polymerase is eukaryal-type family B and/or unique. Other replication-associated proteins are generally considerably more similar to eukaryal than to bacterial counterparts.

Nucleoid segregation and cell division:
Nucleoid segregation mechanisms essentially unknown; few putative partition/mitosis proteins identified. FtsZ-based cell division mechanism in euryarchaea, unknown in other branches.

Transcription:
Eukaryal-type TATA-box-based promoter, TATA-binding protein and transcription factor B. Eukaryal-type multisubunit RNA polymerase. Both eukaryal- and bacterial-type transcription regulators present, bacterial-type sigma factors absent.

Translation:
Bacterial-size ribosomes and rRNA (sequence distinct from bacterial). Bacterial-type ShineDalgarno sequences present, although not in all genes. Non-formylated methionyl-tRNA used in translation initiation. Initiation and elongation factors usually more similar to eukaryal than bacterial counterparts. mRNA capping, splicing and
extensive polyadenylation (probably) absent.

Other properties:
Distinct antibiotic sensitivity spectra. Methanogenesis is unique to the Archaea.

Comments:
Some characteristics are restricted in phylogenetic range. The relative importance of the similarities to either eukaryotes or bacteria depends upon the criteria used. In gene (protein) counting exercises, similarities to bacteria usually dominate over those to eukaryotes. A closer resemblance to eukaryotes is evident when the overall nature of the information processes is considered.