FIGURE 4.

Formate is utilized for thymidylate biosynthesis in purified nuclei. A, nuclei (2 × 10⁷ per condition) were isolated from HeLa cells arrested in S phase with 1 mm HU and released into fresh media for 3 h. The capacity to convert dUMP and [³H]formate to [³H]dTMP was determined in reactions that contained 1) intact nuclei with all the components of the reaction, 2) intact nuclei with the components of the reaction lacking ATP, and 3) intact nuclei with the components of the reaction lacking dUMP as described under “Experimental Procedures.” All reactions were performed in duplicate, and the experiment was repeated two times. Variation is expressed as the S.D. B, nuclei preparation in A was verified by a Western blot with anti-lamin B antibody. Cyt, cytosol; Nuc, nucleus.