PROTEIN-ENCODING GENES IN THE SULFOTHERMOPHILIC ARCHAEA SULFOLOBUS AND PYROCOCCUS

A number of unrelated protein-encoding genes from sulfothermophilic ar chaea, Sulfolobus acidocaldarius, Sulfolobus solfataricus, Pyrococcus furiosus and Pyrococcus woesei, has been analyzed. In the Sulfolobus g enus, the content of A+T is significantly higher than that of C+G and the base usage follows the order, A>T>G>C. In Pyrococcus, the A+T cont ent is also higher than that of C+G, but with lower values; in the ord er of base usage, G precedes T. The codon usage of these sulfothermoph iles has been determined; alternative start codons are frequently used in both genera; codon preferences reflect the rich A+T composition of the corresponding genomes; for both genera the codon bias is particul arly evident within the different arginine triplets, where AGA and AGG are predominant. From the similarities in the codon usage, close taxo nomic relationships become evident within the Sulfolobus or the Pyroco ccus genus; a lower, but significant similarity is also clear between these genera. The synonymous codon usage of these sulfothermophiles sh ows similarities with that of Saccharomyces cerevisiae and bovine mito chondria, whereas clear divergences are observed with the halophilic a rchaeal genus, Halobacterium, or the eubacterium, Escherichia coli. Th e unrelated proteins of the considered sulfothermophiles have been ana lyzed for the content of hydrophobic residues; the comparison with mes ophiles reveals a significant increase in the average hydrophobicity o f amino acid residues. This finding could indicate a mechanism of adap tation of proteins in organisms living under extreme environments. It is noteworthy that an opposite trend, i.e. a decreased average hydroph obicity, occurs in unrelated halophilic proteins.