DETECTION OF PROLIFERATING CELL NUCLEAR ANTIGEN ANALOG IN 4 SPECIES OF MARINE-PHYTOPLANKTON

Proliferating cell nuclear antigen (PCNA) is an auxiliary protein for polymerase-delta and therefore is essential for cellular DNA synthesis. The synthesis and abundance of pcna in the cell are cell-cycle-dependent, both increasing markedly during the S phase. Such a protein could be a useful cell cycle marker, which is required for estimaing algal species-specific growth rates via the cell cycle approach. By using commercially available monoclonal antirat-PCNA antibody and an enhanced chemiluminescence technique, PCNA-like proteins were detected in four species of marine phytoplankton. The strong single band detected on western blots of Isochrysis galbana Parke, Thalassiosira weissflogii Cleve, and Dunaliella tertiolecta butcher had an apparent molecular weight of 33-36 kda. This molecular weight is within the range as observed for PCNA in a wide phylogenetic array of organisms (33-36 kDa). In the diatom Skeletonema costatum (Grev.) Cleve, the PCNA antibody detected a major band of about 19 kDa as well as a minor band of 38 kDa. The detected proteins were specifically recognized by the monoclonal anti-rat-PCNA antibody. The PCNA-like proteins in I. galbana, T. weissflogii, and D. tertiolecta were more abundant in the exponential growth stage and then decreased and became undetectable in the late stationary stage. Our results show that the detected antigens appear to be algal analogs of PCNA.