EXTREMELY THERMOPHILIC AND THERMOSTABLE 5'-METHYLTHIOADENOSINE PHOSPHORYLASE FROM THE ARCHAEON SULFOLOBUS-SOLFATARICUS - GENE CLONING AND AMINO-ACID-SEQUENCE DETERMINATION

A gene encoding an extremely thermophilic and thermostable 5'-methylth ioadenosine phosphorylase was cloned from the archaeon Sulfolobus solf ataricus. Two degenerate oligodeoxyribonucleotide probes synthesized o n the basis of the N-terminal amino acid sequence of the protein were used to screen a mic library of S. solfataricus cloned into the pGEM7Z f(+) vector. The DNA fragment of 2118 bp containing the 5'-methylthioa denosine phosphorylase gene was sequenced. The open reading frame comp rises 711 nucleotides, which includes the stop codon; and encodes a pr otein of 236 residues whose molecular mass is in good agreement with t he value determined by gel filtration for the purified enzyme, The N- and C-terminal sequences of the protein and the sequences of the pepti des prepared by cyanogen bromide cleavage exactly match with the corre sponding sequences deduced from the gene, thus confirming the identity of the 5'-methylthioadenosine phosphorylase gene. Typical archaebacte rial regulatory sites were identified in the flanking regions and a po tential Shine-Dalgarno-like sequence was recognized around the ATG ini tiation codon, The deduced amino acid sequence showed 32% identity and 30% identity with Escherichia coli purine-nucleoside phosphorylase an d with E. coli uridine phosphorylase, respectively, Evolutionary and s tructural implications of this similarity are discussed.