Archaeocin is the name given to a new type of potentially useful antibiotic that is derived from the Archaea group of organisms.^[1] Eight archaeocins have been partially or fully characterized, but hundreds of archaeocins are believed to exist, especially within the haloarchaea. Production of these archaeal proteinaceous antimicrobials is a nearly universal feature of the rod-shaped haloarchaea.^[2]

The prevalence of archaeocins from other members of this domain is unknown simply because no one has looked for them. The discovery of new archaeocins hinges on recovery and cultivation of archaeal organisms from the environment. For example, samples from a novel hypersaline field site, Wilson Hot Springs in the Fish Springs National Wildlife Refuge in eastern Utah,^[3] recovered 350 halophilic organisms; preliminary analysis of 75 isolates showed that 48 were archaeal and 27 were bacterial.^[4]

Halocins are classified as either peptide (≤ 10 kDa; 'microhalocins') or protein (> 10 kDa) antibiotics produced by members of the archaeal family Halobacteriaceae. To date, all of the known halocin genes are encoded on megaplasmids (> 100 kbp) and possess typical haloarcheal TATA and BRE promoter regions. Halocin transcripts are leaderless and the translated preproteins or preproproteins are most likely exported using the twin arginine translocation (Tat) pathway, as the Tat signal motif (two adjacent arginine residues) is present within the amino terminus. Halocin genes are almost universally expressed at the transition between exponential and stationary phases of growth; the only exception is halocin H1, which is induced during exponential phase. In contrast, the larger halocin proteins are heat-labile and typically obligately halophilic as they lose their activity (or activity is reduced) when desalted.^{[citation needed]}

The archaeocins produced by Sulfolobus are entirely different from halocins, since their activity is predominantly associated with the cells and not the supernatant. To date, the spectrum of sulfolobicin activity appears to be restricted to other members of the Sulfolobales: the sulfolobicin inhibited S. solfataricus P1, S. shibatae B12, and six nonproducing strains of S. islandicus. Activity appears to be archaeocidal but not archaeolytic.^[4] Two genes involved in sulfolobicin production have been identified in S. acidocaldarius and S. tokodaii. The sulfolobicins appear to represent a novel class of antimicrobial proteins.^[7]

• Archaea