Characterization of a b-1,3-Glucanase Encoded by Chlorella Virus PBCV-1

Sequence analysis of the 330-kb chlorella virus PBCV-1 genome revealed an open-reading frame, A94L, that encodes a protein with significant amino acid identity to Glycoside Hydrolase Family 16 b-1,3-glucanases. The a94l gene was cloned and the protein was expressed as a GST-A94L fusion protein in Escherichia coli. The recombinant A94L protein hydrolyzed the b-1,3-glucose polymer laminarin and had slightly less hydrolytic activity on b-1,3-1,4-glucose polymers, lichenan and barley b-glucan. The recombinant enzyme had the highest activity at 65°C and pH 8. We predicted that the a94l-encoded b-1,3-glucanase is involved in degrading the host cell wall either during virus release and/or is packaged in the virion particle and involved in virus entry. Therefore, we expected a94l to be expressed late in virus infection. However, contrary to expectations, both the a94l mRNA and the A94L protein appeared 15 min after PBCV-1 infection and disappeared 60and 120-min p.i. postinfection, respectively, indicating that a94l is an early gene. Twenty-seven of 42 chlorella viruses contained a virus