PCNA

Proliferating cell nuclear antigen (PCNA) is an accessory protein to DNA polyerase delat and epsilon required for DNA replication. It is considered a "clamp" to enhance the processivity of the DNA polymerase. First found in human patients of autoimmune disease in 1985, this protein turns out to be ubiquitous and highly conserved. It has been detected in virus, archea and all eukaryotes. PCNA has been found to be associated with the S phase of the cell cycle. In eukaryotic cells, a cell undergoes G1 (gap or growth stage 1), S (DNA replication), G2 (gap or growth stage 2), and M (mitosis) phases leading to cell division.

In 1994, we first detected PCNA in marine phytoplankton. To date, PCNA and its encoding gene has been detected and characterized in a variety of phytoplankton taxa: chlorophyceae, bacillariophyceae, haptophyceae, cryptophyceae, pelagophyceae, and dinophyceae. We study phytoplankton PCNA because it can be used as a cell cycle marker for estimation of cell division rate in the environment. The underlying principle is that the completion of each round of the cell cycle results in growth of the population. Monitoring the progression of the cell cycle through a segment of the cell cycle enar the M phase will allow one to calculate growth rate based on the time sequential values of the fraction of the cell population residing in this segment and the duration of the segment. Although PCNA marks the S phase, not quite near the M phase, our theoretical analysis shows that it can still provides a good approximation of in situ growth rate.