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 exsymbiont green algae Chlorella strain NC64A. The viral, 330-kb DNA genome encodes a relatively large number of functionally active proteins including restriction and modification enzymes, DNA polymerase, glycosylation, and cell wall degrading enzymes. Sequencing of the viral DNA, now in progress, revealed many major open reading frames (ORF), which resemble known genes in sequence data bases and which have not previously been found in viral genomes. Here we report on the identification and characterization of one such gene, aspartate transcarbamylase (ATCase), an enzyme that catalyzes the committing step in the de novo biosynthetic pathway of pyrimidines. The cloned gene is highly homologous to a variety of plant ATCases and includes the typical ATCase catalytic motif. When cloned into the pGEX-2T expression vector, a fusion protein with ATCase activity could be demonstrated and distinguished from the host ATCase activity. The viral enzyme is expressed early and transiently in the infection. To our knowledge, this is the first virus known to encode and express its own de novo nucleotide precursors' synthetic enzymes.

Selective killing of transformed rat cells by minute virus of mice does not require infectious virus production.

Fischer rat fibroblasts, naturally resistant to killing by the fibrotropic strain of minute virus of mice [(parvovirus MVM(p)], became sensitive to MVM when transformed by polyomavirus. This sensitization did not involve an increase in the percentage of cells which synthesized viral capsid antigens or in the percentage of cells which produced infectious virus. The addition of anti-MVM antiserum to the growth medium of MVM-infected cells had only a small effect on their survival rates, indicating that the majority of the killing effect of MVM occurs in a single cycle of infection. The data indicate that cell killing by MVM is independent of infectious virus production and thus support the notion that the preferential cytolytic effect is affected by viral cytotoxic gene products which accumulate to intolerable levels in transformed cells but not in normal ones. Finally, using cells transformed with polyomavirus and genomic and subgenomic clones of polyomavirus, we showed that the extent of sensitization to killing by MVM depended on the transforming agent used.