The similarity of the morphology and of DNA composition, the homology of the component patterns of DNA-dependent RNA polymerases and their immunochemical crossreactivity support the conclusion that several extreme thermoacidophiles are related to each other. We name two new species of the genus Sulfolobus. The first, Sulfolobus solfataricus (DSM 1616 and DSM 1617) has the same GC content in its DNA and the same general properties as S. acidocaldarius, but differs significantly from the latter species in the molecular weights of the 11 components of its RNA polymerase and in the salt requirements of this enzyme. The second, Sulfolobus brierleyi, DSM 1651, differs from S. acidocaldarius in several respects. The cells show much less stability at neutral pH. The GC content is significantly lower. The RNA polymerase lacks two components present in the enzymes from the other species. The residual 9 components show larger size differences from the homologous subunits of the S. acidocaldarius enzyme.
Like the enzyme from S. solfataricus, the polymerase from S. brierleyi yields an incomplete immunochemical crossreaction with an antibody against the RNA polymerase from S. acidocaldarius.
The isolates DSM 1616 and DSM 1617 of Sulfolobus solfataricus are probably identical with or similar to the “Caldariella” strains MT 3 and MT 4, isolated by de Rosa et al. (1975).
Like all other known archaebacterial RNA polymerases the enzymes from these species are insensitive to rifampicin and streptolydigin.