DNA Topoisomerases I From Thermophilic Bacteria: Cloning and Sequencing of the DNA Topoisomerase I from Fervidobacterium islandicum

Summary The gene encoding the DNA topoisomerase I from the thermophilic bacterium Fervidobacterium islandicum has been cloned by using oligonucleotides derived from internal microsequencing of the purified protein ( Bouthier de la Tour et al., 1993). The corresponding nucleotide sequence has been determined, revealing an open reading frame of 2097 nucleotides. The deduced amino acid sequence (699 residues) clearly showed that the gene belongs to E. coli TopA family, and has 53% identity with Thermotoga maritima TopA ( Bouthier de la Tour et al., 1995). Comparison of the F. islandicum sequence to the topoisomerase I sequences of other species revealed several interesting features: (1) a putative N-terminal tail of 33 amino acids, whose function is unknown. (2) a 8 amino acids insertion close to the active site tyrosine, shared by 3 other bacterial TopA sequences. (3) a unique putative zinc finger that is conserved in 7 out of 9 sequences. However, no significant feature linked to thermophily could distinguish the two thermotogales topoisomerases from their mesophilic counterparts. Finally, the analysis of the gene organization in the vicinity of the TopA locus in different bacterial species showed that it was not conserved.