Cell cycle-dependent modulation of O6-methylguanine-DNA methyltransferase in C3H/10T1/2 cells.

O6-methylguanine-DNA methyltransferase (MGMT) was measured in partially synchronized cultures of C3H/10T1/2 mouse embryo cells as a function of cell cycle. The degree of synchrony and progression of the cell cycle were monitored by flow cytometry. The MGMT level was significantly reduced prior to the onset of S-phase. This reduction was concomitant with the inhibition of in vivo repair of O6-methylguanine in DNA of S-phase cells as observed earlier. The recovery of the MGMT level paralleled the progression of synchronized cells into G2. S-phase cells purified by cell sorting contained approximately 15% of the MGMT present in G0 or early G1 cells. A comparison of the in vivo repair of O6-methylguanine and MGMT levels suggests that the lack of repair of O6-methylguanine in DNA of the mouse embryo cells is due only in part to a temporal loss of MGMT.