TEMPERATURE-SENSITIVE MUTATION OF DNA-POLYMERASE-ALPHA INDUCES GROWTH-SUPPRESSIVE PHENOTYPES INVOLVING RETINOBLASTOMA PROTEIN AND CYCLIN D1

Temperature-sensitive (ts) cell cycle mutant mouse cell, tsFT20, is de ficient in DNA polymerase a activity to initiate DNA replication at re plicon origins. Here, we analyzed phenotypes concerning growth control genes in the arrested tsFT20 cells. Analysis of cyclins showed that e xpression levels of cyclin D1, which is essential for G(1)/S transitio n, remarkably decreased in the mutant cells after temperature up-shift . Further we examined phosphorylation states of retinoblastoma protein (pRB) in the cells. Though the tsFT20 cells arrested in G(1)/S-S phas e at nonpermissive temperature (Eki et al., (1990) J. Biol. Chem. 265 26-33), a large proportion of pRB was found as an underphosphorylated growth-suppressive form in the arrested cells. In revertant cell lines of tsFT20, pRB was not underphosphorylated even at nonpermissive temp erature. The pRB underphosphorylation occurred later than the decrease of mRNA levels of cyclin D1, thus the underphosphorylation may be cau sed by the decrease in amount of cyclin D1 protein. These results indi cated that the mutational inactivation of DNA polymerase a evokes phen otypes in which the inhibitory machinery of G(1)/S transition has been turned on.