Spherical particles (SPs) of approximately 30 nm in diameter were found in the hyperthermophilic archaeon Pyrococcus furiosus. The SPs contained no nucleic acid and were composed of a single 39-kDa protein. The amino acid sequences of the amino-terminal and internal fragments were identical to portions of the deduced amino acid sequence of the putative 38.7-kDa protein encoded by the genome of P. furiosus, suggesting that the protein was expressed from the genome of P. furiosus. This possibility was confirmed by the observation that the 38.7-kDa protein expressed in Escherichia coli reacted specifically with the antibody against purified SPs, and it also formed SPs similar to those found in P. furiosus. Of the 345 amino acid residues in the 38.7-kDa protein, the amino-terminal 100 amino acids exhibited strong homology to putative proteins from other species of Pyrococcus, while the remaining 245 carboxy-terminal residues were not significantly homologous to putative proteins from other members of archaea. Thus, the carboxy-terminal region might be the product of a foreign gene that was incorporated relatively recently into the genome of P. furiosus.
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