Dissociation of nuclear DNA replication from concomitant protein synthesis in synchronous cultures of chlorella

Abstract

The interrelationship between nuclear DNA replication and cytoplasmic protein synthesis in Chlorella was studied by use of hydroxyurea and cycloheximide as specific inhibitors. 15 μM cycloheximide completely blocked the synthesis of protein and depressed DNA and RNA formation by about 90 %. Protein synthesis resumed immediately after removal of the inhibitor, while DNA synthesis was fully restored after a lag time of about 2 h. 10 mM hydroxyurea completely inhibited DNA synthesis. A decrease in protein and RNA synthesis gradually ensued after about 3 h of exposure. DNA synthesis was restored immediately after removal of the inhibitor, but the final DNA levels were markedly lower than in control cultures. During the recovery from the hydroxyurea treatment about 50 % increase of the DNA content was observed in the presence of cycloheximide. The results suggest that simultaneous synthesis of specific proteins required for DNA replication occurs during the S phase. These proteins continue to be synthesized when nuclear DNA replication is stopped by specific inhibitors, and can then be used to support subsequent DNA replication in the absence of protein synthesis.