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Isolation of thymidine-requiring variants from rat nerve-like cells.

Thymidine auxotrophs (B3T) of rat nerve-like cells (B103) were isolated. B103 cells were preincubated in a thymidine medium and mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Afterward, cells were incubated in a medium supplemented with dialyzed fetal calf serum and then treated with 5-fluorodeoxyuridine for 24 h. Next, cells were grown in a thymidine medium for twenty days. Thymidine auxothrophs were thus obtained and one clone of them was designated B3T cells. The modal number of chromosomes in B3T cells was 85. Growth tests revealed the following interesting facts: (1) The medium containing pyrazofurin and uridine could not support the growth of B3T cells, but the addition of thymidine to such a medium enabled cells to grow. (2) Even deoxyuridine supported the growth of B3T cells when added to the medium of pyrazofurin and uridine. These findings suggest that the catalytic ability of thymidylate synthetase in B3T cells may have decreased, probably due to the decreased affinity of the enzyme molecules to the substrate (dUMP), and that thymidine kinase activity was high enough to support the growth of B3T cells. B3T cells have maintained the ability to differentiate and extend neurites in response to dibutyryl-cyclic AMP as also demonstrated in wild cells (B103). B3T cells will be available for genetical and molecular biological studies of neuronal cells.