A library of double-stranded cDNA was constructed from ts13 cells, a Gl-specific temperature-sensitive hamster cell line. The cDNAs, cloned into pBR322, were prepared from poly(A)+ mRNA isolated from ts13 cells 6 hr after serum stimulation at the permissive temperature of 34°C.Differential screening of the library with G -specific and Gospecific single-stranded cDNA probes prepared from the same cells identified five cDNA clones whose sequences were preferentially expressed in G1. (2), is a mutant of RNA polymerase II (3-6). In addition, the progression of cells in culture through G1 can be inhibited by the intracellular microinjection of a-amanitin (5), a drug that at low concentrations is known to have one and only one specific site of action, the large subunit of RNA polymerase II (7). These studies conclusively showed that a functional RNA polymerase II is an absolute requirement for the transition of cells from a resting to a growing state. This requirement justifies the search for genes transcribed by RNA polymerase II that control the transition from Go to G1 to S. As a first step in this direction, we have begun a search for genes whose expression is specifically increased in G1. A cDNA library was constructed from poly(A)+ mRNA isolated from serumstimulated ts13 cells that are a G1-specific temperature-sensitive mutant of the cell cycle (2,8). Genes that were preferentially expressed in G1 were identified by differential hybridization. The combination of molecular biology techniques with the use of Gl-specific temperature-sensitive mutants should ultimately allow us to select among the preferentially expressed genes those that play a major role in cell-cycle progression. This initial report describes the identification and characterization of five cDNA clones that are preferentially expressed in G1 at the permissive temperature yet are differently affected by the temperature-sensitive block at the restrictive temperature.
MATERIALS AND METHODSCell Lines and Culture Conditions. The cell lines ts13 and tsAF8 are G1-specific temperature-sensitive mutants originally isolated from baby hamster kidney (BHK) cells (2, 8). Both lines were maintained under culture conditions described in detail (9, 10). The permissive temperature for both cell lines was 34WC, and the nonpermissive temperature was 39.60C for ts13 and 40'C for tsAF8. Cells were made quiescent by serum deprivation [i.e., maintained 48-50 hr in Dulbecco's minimal essential medium (DME medium) supplemented with 0.5% calf serum]. Quiescent populations were stimulated with fresh DME medium containing 15% fetal calf serum. The entry of cells into DNA synthesis was routinely monitored by continuous labeling with [3H]thymidine (6.7 Ci/mmol; 0.5 ,Ci/ml; 1 Ci = 37 GBq; New England Nuclear) followed by autoradiography.Enzymes. All restriction endonucleases were purchased from Bethesda Research Laboratories or New England Biolabs and were used according to the manufacturers' directions. Avian myeloblastosis virus reverse transcriptase was provided by...