AN EARLY GENE OF THE CHLORELLA-VIRUS PBCV-1 ENCODES A FUNCTIONAL ASPARTATE-TRANSCARBAMYLASE

PBCV-1 belongs to a family of large viruses that replicate in the exsy mbiont green algae Chlorella strain NC64A. The viral, 330-kb DNA genom e encodes a relatively large number of functionally active proteins in cluding restriction and modification enzymes, DNA polymerase, glycosyl ation, and cell wall degrading enzymes. Sequencing of the viral DNA, n ow in progress, revealed many major open reading frames (ORF), which r esemble known genes in sequence data bases and which have not previous ly been found in viral genomes. Here we report on the identification a nd characterization of one such gene, aspartate transcarbamylase (ATCa se), an enzyme that catalyzes the committing step in the de novo biosy nthetic pathway of pyrimidines. The cloned gene is highly homologous t o a variety of plant ATCases and includes the typical ATCase catalytic motif. When cloned into the pGEX-2T expression vector, a fusion prote in with ATCase activity could be demonstrated and distinguished from t he host ATCase activity. The viral enzyme is expressed early and trans iently in the infection, To our knowledge, this is the first virus kno wn to encode and express its own de novo nucleotide precursors' synthe tic enzymes. (C) 1996 Academic Press, Inc.