Thymidine kinases in archaea.

Twenty-six fully sequenced archaeal genomes were searched for genes coding for putative deoxyribonucleoside kinases (dNKs). We identified only 5 human-like thymidine kinase 1 genes (TK1s) and none for non-TK1 kinases. Four TK1s were identified in the Euryarchaea and one was found in the Crenarchaea, while none was found in Nanoarchaeum. The identified TK1s have high identity to Gram-positive bacteria TK1s. The TK1s from archaea, Gram-positive bacteria and eukaryotes share the same common ancestor, while the TK1s from Gram-negative bacteria belong to a less-related subgroup. It seems that a functional deoxyribonucleoside salvage pathway is not crucial for the archaeal cell.