Interference in hybrid clone selection caused by Mycoplasma hyorhinis infection

Mycoplasma hyorhinis strains were isolated from Chinese hamster DON cells which lacked the ability to produce hybrid colonies in HAT medium. The mycoplasma isolates were virtually devoid of HGPRT activity in vivo and in vitro in the presence of excess co-enzyme, phosphoribosylpyrophosphate. Deliberate infection of mycoplasma-free cells caused no alterations in the HGPRT^? and TK^? phenotypes of the cells. Heterokaryon formation with infected cells was normal and the failure to produce hybrid colonies resulted from depletion, by nucleoside phosphorylase activity, of exogenous thymidine required for rescue of hybrid cells in HAT medium. Increasing the thymidine concentration and repeatedly replenishing HAT medium permitted hybrid clone formation.