Abstract
Chloroviruses are large double-stranded DNA (dsDNA) viruses that infect certain isolates of chlorella-like green algae. They contain up to approximately 400 protein-encoding genes and 16 transfer RNA (tRNA) genes. This review summarizes the unexpected finding that many of the chlorovirus genes encode proteins involved in manipulating carbohydrates. These include enzymes involved in making extracellular polysaccharides, such as hyaluronan and chitin, enzymes that make nucleotide sugars, such as GDP-l-fucose and GDP-d-rhamnose and enzymes involved in the synthesis of glycans attached to the virus major capsid proteins. This latter process differs from that of all other glycoprotein containing viruses that traditionally use the host endoplasmic reticulum and Golgi machinery to synthesize and transfer the glycans.
Highlights
In discussing enzymes involved in manipulating carbohydrates, one usually does not consider viruses to play a role in this important subject
As described in this review, chloroviruses that infect certain isolates of single-celled, eukaryotic chlorella-like green algae are an exception to this process because they encode enzymes involved in making extracellular polysaccharides, nucleotide sugars and the synthesis of glycans attached to their major capsid glycoproteins
We have considered the following possible evolutionary advantages for acquiring these genes: (1) the polysaccharides prevent infection by a second chlorovirus; (2) they cause the infected cells to clump with uninfected host cells, increasing the probability that progeny viruses can infect healthy host cells; (3) they prevent paramecia from taking up infected algal cells, (4) the chloroviruses have another host in nature, and this other host is attracted to or binds to hyaluronan or chitin on virus-infected algae, which would facilitate progeny-virus infections; or (5) polysaccharides increase the functional diameter of the infected cell, which might facilitate consumption by a predator
Summary
In discussing enzymes involved in manipulating carbohydrates, one usually does not consider viruses to play a role in this important subject. DNA (dsDNA)-containing viruses (genomes of 290 to 370 kb) with an internal lipid membrane. They exist in inland waters throughout the world with titers occasionally reaching thousands of plaque-forming units (PFU) per mL of indigenous water. Known chlorovirus hosts, which are normally endosymbionts and are often referred to as zoochlorellae [1,2], are associated either with the protozoan Paramecium bursaria, the coelenterate Hydra viridis or the heliozoan Acanthocystis turfacea [3,4,5,6]. PBCV-1 and other chlorovirus genomes contain methylated bases, which occur in specific DNA sequences. We have listed putative chlorovirus genes involved in carbohydrate metabolism, which are encoded by the 43 chloroviruses whose genomes have been sequenced, in Table 1, Table 2 and Table 4.
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