LOCALIZATION OF HISTONE-LIKE PROTEINS IN THERMOPHILIC ARCHAEA BY IMMUNOGOLD ELECTRON-MICROSCOPY

Immunogold staining combined with heavy metal-based DNA immunostaining of ultrathin sections of cryofixed, freeze-substituted, and resin-emb edded cells has revealed that in the hyperthermophilic archaeon, Sulfo lobus acidocaldarius, the histone-like proteins HSNP-A and HSNP-C' are colocalized with DNA in the ribosome-free nucleoid, whereas DBNP-B is located exclusively in the ribosome-containing cytoplasm. Following c hemical fixation and solvent dehydration the chromatin was resistant t o aggregation, indicative of a high protein content. Immunogold labeli ng of chromatin released from isolated nucleoids of S. acidocaldarius demonstrated that the helix-stabilizing protein HSNP-C' was randomly d istributed on both filamentous DNA and on more globular material prese nt within distinct nucleoid subdomains. DNA-dependent RNA polymerase m olecules were located by immunogold staining predominantly in the cyto plasmic region immediately surrounding the nucleoid in S. acidocaldari us, presumably at the locations of active transcription. Affinity-puri fied antibodies raised against HMf, a histone-like protein isolated fr om the hyperthermophilic and only distantly related methanogenic archa eon Methanothermus fervidus, were also found to be localized almost ex clusively to the DNA-containing, ribosome-free nucleoid region. (C) 19 94 Academic Press, Inc.