Regulation of folate reductase synthesis in sensitive and methotrexate-resistant sarcoma 180 cells. In vitro translation and characterization of folate reductase mRNA.

A highly specific assay for folate reductase mRNA activity from Sarcoma 180 cells was developed using the rabbit reticulocyte lysate protein synthesizing system. Quantitation of in vitro folate reductase synthesis was accomplished by direct immunoprecipitation from lysate reactions. The in vitro labeled folate reductase was synthesized in a linear response to a wide range of RNA concentrations, migrated as a single prominent radioactive species upon polyacrylamide gel electrophoresis, and was indistinguishable from authentic 14C-labeled folate reductase on the basis of molecular weight and immunotitration with anti-folate reductase gamma-globulin. The assay was used to quantitate folate reductase mRNA activity in various cell lines and under several conditions known to affect folate reductase synthesis. These included (a) sensitive and methotrexate-resistant Sarcoma 180 cells, (b) two lines of resistant cells having different relative rates of folate reductase synthesis, (c) growth of methotrexate-resistant cells in the absence of methotrexate, and (d) growth phase. The results indicate that the relative rate of folate reductase synthesis in each case can be explained solely by the level of translatable folate reductase mRNA. The use of poly(U)-Sepharose and sucrose gradient fractionation procedures indicated that folate reductase mRNA contains poly(A) and has a sedimentation coefficient of approximately 14 S. These two fractionation steps were combined to achieve an approximately 90-fold purification of folate reductase mRNA over total cytoplasmic RNA.