DNA sequence analysis of the 330-kb Chlorella virus PBCV-1 genome unexpectedly revealed several open reading frames which encode proteins that are homologous to sugar-manipulating enzymes including glutamine:fructose-6- phosphate amidotransferase (GFAT), UDP-glucose dehydrogenase (UDP-GlcDH), and hyaluronan synthase (HAS). PBCV-1 genes encoding the putative GFAT and UDP- GlcDH enzymes were expressed in Escherichia coli and both recombinant proteins have the predicted enzyme activity in cell free extracts. These same two genes are transcribed early in PBCV-1 infection, and both genes are widespread among the Chlorella viruses. The products of the reactions catalyzed by these two enzymes are precursors in the biosynthesis of hyaluronan polysaccharide. Previous experiments established that, like the GFAT and UDP-GlcDH genes, the HAS gene is transcribed early and encodes a functional enzyme (DeAngelis, P.L., Jing. W., Graves, M.V., Burbank, D.E., and Van Etten, J.L. (1997) Science 278, 1800-1803). Interestingly, the predicted amino-acid sequences of the PBCV-1 GFAT and UDP-GlcDH enzymes are more similar to bacterial GFAT and UDP-GlcDH enzymes than to their eukaryotic counterparts. In contrast, the amino-acid sequence of the PBCV-1 HAS enzyme more closely resembles eukaryotic enzymes.
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