Forty-three kb of DNA, located at the left end (45 to 88 kb) of the 330-kb Chlorella virus PBCV-1 genome, was sequenced and analyzed. Eighty-six open reading frames (ORFs) 65 codons or longer were identified; 47 were classified as major ORFs. These 47 major ORFs are densely packed on both strands of PBCV-1 DNA. Seventeen of these major ORFs resemble genes in the sequence databases, including three putative gene products involved in manipulating sugars (glucosamine synthetase, GDP-D-mannose dehydratase, and N -acetylglucosaminyltransferase), two transcription factors, β-1,3-glucanase, aspartate transcarbamylase, ubiquitin carboxy terminal hydrolase, RNA guanyl transferase, an exonuclease, and a helicase. This is the first time some of these putative PBCV-1 genes have been found in a virus genome. One of the transcription factor-like genes contains a type IB self-splicing intron. Since a spliceosomal processed intron was reported previously in the PBCV-1 DNA polymerase gene, PBCV-1 is the first virus known to contain two different types of introns.
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