Cell cycle markers for live cell analyses

Many cellular processes are regulated by cell cycle dependent changes in protein dynamics and localization. Studying these changes in vivo requires methods to distinguish the different cell cycle stages. Here we demonstrate the use of DNA Ligase I fused to DsRed1 as an in situ marker to identify S phase and the subsequent transition to G₂ in live cells. Using this marker, we observed changes in the nuclear distribution of Dnmt1 during cell cycle progression. Based on the different nuclear distribution of DNA Ligase I and Dnmt1 in G₂ and G₁, we demonstrate that the combination of both proteins allows the direct discrimination of all cell cycle phases using either immunostainings or fusions with fluorescent proteins. These markers are new tools to directly study cell cycle dependent processes in both, fixed and living cells.