Semi-conservative and non-conservative replication of DNA in temperature-sensitive mouse L-cells

The mode of DNA replication has been studied in wild-type mouse L-cells (WT-4) and in two subclones (TS A1S9 and ts C1 cells) which are temperature-sensitive in DNA synthesis. It has been demonstrated that DNA is replicated by the semi-conservative mechanism in WT-4 cells grown at 34 degrees C or at 38.5 degrees C throughout the logarithmic phase and into the stationary phase. Similar results were obtained with ts A1S9 and ts C1 cells grown at the permissive temperature (34 degrees C). When the latter cells were incubated at the non-permissive temperature (38.5 degrees C) inactivation of DNA synthesis appeared to proceed through three general stages. During the first 24 h after temperature upshift suppression of semi-conservative DNA replication occurred. During the second stage a very low level of semi-conservative synthesis was maintained. During the third stage, incorporation of dThd into DNA began to increase, often reaching 10-20% of control levels after 3-5 days. During this third stage DNA synthesis was effected by a non-conservative mechanism. Temperature-inactivated ts A1S9 cells and ts C1 cells were able to perform semi-conservative synthesis upon back-shift to 34 degrees C, using as template that DNA synthesized prior to temperature upshift.