Inhibition of [3H]thymidine incorporation by Mycoplasma arginini‐infected cells due to enzymatic cleavage of the nucleoside

Culture supernatants contaminated by Mycoplasma arginini inhibit the incorporation of [3H]thymidine ([3H]dThd) by cytotoxic T lymphocyte cell lines. This study presents evidence that the inhibition of uptake of the nucleoside is due to the rapid cleavage of the exogenous [3H]dThd into thymine. Undine and cytidine as well as dThd are degraded by the mycoplasma‐contaminated supernatants, while no cleavage was observed with uninfected supernatants. Cells contaminated by mycoplasma apparently release a pyrimidine‐specific nucleosidase, possibly a dThd phosphorylase, which is responsible for the inhibition.