Description

PCNA (proliferating cell nuclear antigen) is the DNA polymerase processivity factor that loads onto the chromatin during S phase of the cell-cycle (Brauchle et al. 2003). Thus, nuclear localization of PCNA (PCN-1 in C. elegans) is used as a marker for the S phase of the cell cycle (Brauchle et al. 2003). GFP::PCN-1 has been shown to label S phase in C. elegans embryo  when driven through the germline and embryonic promoter pie-1 (Brauchle et al. 2003). We assayed GFP::PCN-1 (allele isIs17, GZ264 (Brauchle et al. 2003)) as a marker for S phase in adult germline progenitor zone cells. If this reagent were a faithful marker of S phase in germline progenitor zone cells, we would expect nuclear localization during S phase, and nuclear exclusion in the other phases of the cell-cycle, as is the case in the C. elegans embryo. We would also expect a perfect overlap with EdU which marks S phase of the cell cycle. EdU is incorporated in ~55-60% of the adult hermaphroditic wild-type progenitor zone cells (Fox et al. 2011; Furuta et al. 2018). We found that GFP::PCN-1 was nuclear in almost all of the progenitor zone cells, irrespective of whether they were EdU positive or EdU negative (arrowhead, Figure 1). The only cells that excluded GFP::PCN-1 from the nucleus were the metaphase cells (arrow) during M phase, when the nuclear envelope breaks down. Thus, GFP::PCN-1 does not overlap with EdU labeling in adult progenitor zone cells. These data suggest that nuclear localization of GFP::PCN-1 is not a good marker of S phase dynamics in the C. elegans adult germline progenitor cells. This could either be because GFP signal perdures in the nucleus in this context, or that GFP::PCN-1 is nuclear localized throughout the cell cycle in germline progenitor zone cells, unlike in the C. elegans embryo.