Periodic Increase in Deoxyribonuclease Activity during the Cell Cycle in Synchronized Euglena

The deoxyribonucleases have numerous biological functions in cellular growth and development (1-3) . These include repair mechanisms for damaged DNA molecules, protective mechanisms against viral infection, promotion by viral DNases of intracellular viral growth, and intracellular digestive functions carried out by acid nucleases located in the lysosomes . Accumulating evidence indicates that another possible function of DNases is in the control of DNA synthesis . Stern (4) has found a sharp periodicity of DNase activity in lily anthers followed by the appearance of a pool of deoxyribosides. Escherichia coli mutants deficient in endonuclease I failed to utilize deoxynucleoside triphosphates for DNA synthesis (5) ; and nuclease activity has been found associated with, or necessary for, DNA polymerase activity (6-8) . Indeed, both nuclease and polymerase activities have recently been shown to be present on the same molecules, suggesting a subunit structure (9) . Although nuclease activity has been shown in bacteria, higher plants, and animal tissues, relatively little work has been done with algae (10) . Since Euglena can be synchronized by appropriate light/dark cycles and has been shown to synthesize its DNA at a specific time during the cell cycle (11, 12), this organism seemed a useful tool for studying the activity of DNases and their temporal association with DNA synthesis . This note reports that DNA synthesis in Euglena occurs in a discontinuous manner beginning approximately 6-7 hr after the onset of the light period, while DNase activity increases approximately at the time of DNA synthesis . Column chromatography indicates that only one active species of DNase exists, having a pH optimum of 7 .5 . This temporal relation between DNA synthesis and DNase activity suggests a possible control mechanism by the latter and is discussed in relation to evidence in other organisms .