Type IA topoisomerases have highly conserved catalytic N-terminal domains for the cleaving and rejoining of a single DNA/RNA strand that have been extensively characterized. In contrast, the C-terminal region has been less covered. Two major types of small tandem C-terminal domains, Topo_C_ZnRpt (containing C4 zinc finger) and Topo_C_Rpt (without cysteines) were initially identified in Escherichia coli and Mycobacterium tuberculosis topoisomerase I, respectively. Their structures and interaction with DNA oligonucleotides have been revealed in structural studies. Here, we first present the diverse distribution and combinations of these two structural elements in various bacterial topoisomerase I (TopA). Previously, zinc fingers have not been seen in type IA topoisomerases from well-studied fungal species within the phylum Ascomycota. In our extended studies of C-terminal DNA-binding domains, the presence of zf-GRF and zf-CCHC types of zinc fingers in topoisomerase III (Top3) from fungi species in many phyla other than Ascomycota has drawn our attention. We secondly analyze the distribution and combination of these fungal zf-GRF- and zf-CCHC-containing domains. Their potential structures and DNA-binding mechanism are evaluated. The highly diverse arrangements and combinations of these DNA/RNA-binding domains in microbial type IA topoisomerase C-terminal regions have important implications for their interactions with nucleic acids and protein partners as part of their physiological functions.
Keywords: Top3; TopA; topoisomerase; type IA; zf-CCHC; zf-GRF; zinc fingers.