Cell cycle-dependent modulation of o-methylguanine-dna methyltransferse 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 -15% of the MGMT present in G_o 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.