The three-domain uridine kinase (TmUK) was described before to be a result of interkingdom gene fusions, which is shared by Treponema pallidum and T. maritima . Using the full-length protein, no UK activity was observed in this study (Figure ). However, ATP phosphatase activity was observed in the presence of uridine (Urd). Therefore, we decided to study if truncated versions (TmUKs1-4, Figure ) only coding for the UK domain would show Urd phosphorylation activity. Truncated UK variants were cloned, heterologously expressed in E. coli and purified by affinity chromatography. While TmUKs1-3 were produced in substantial amount and with good purity, TmUKs4 could not be produced since only insoluble protein was obtained. Activity testing with Urd as substrate revealed no formation of UMP with all truncated UK variants. These results together with the observed ATP phosphatase activity in the presence of Urd fit well to the hypothesis that the three-domain UK of Treponema pallidum and T. maritima are involved in the in autoregulation of translation .