This report completes a preliminary analysis of the sequence of the 330,740-bp chlorella virus PBCV-1 genome, the largest virus genome to be sequenced to date. The PBCV-1 genome is 57% the size of the genome from the smallest self-replicating organism, Mycoplasma genitalium. Analysis of 74 kb of newly sequenced DNA, from the right terminus of the PBCV-1 genome, revealed 153 open reading frames (ORFs) of 65 codons or longer. Eighty-five of these ORFs, which are evenly distributed on both strands of the DNA, were considered major ORFs. Fifty-nine of the major ORFs were separated by less than 100 bp. The largest intergenic distance was 729 bp, which occurred between two ORFs located in the 2.2-kb inverted terminal repeat region of the PBCV-1 genome. Twenty-seven of the 85 major ORFs resemble proteins in databases, including the large subunit of ribonucleotide diphosphate reductase, ATP-dependent DNA ligase, type II DNA topoisomerase, a helicase, histidine decarboxylase, dCMP deaminase, dUTP pyrophosphatase, proliferating cell nuclear antigen, a transposase, fungal translation elongation factor 3 (EF-3), UDP glucose dehydrogenase, a protein kinase, and an adenine DNA methyltransferase and its corresponding DNA site-specific endonuclease. Seventeen of the 153 ORFs resembled other PBCV-1 ORFs, suggesting that they represent either gene duplications or gene families.
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