Characterization of a new chlorovirus type with permissive and non-permissive features on phylogenetically related algal strains

Abstract

A previous report indicated that prototype chlorovirus PBCV-1 replicated in two Chlorella variabilis algal strains, NC64A and Syngen 2-3, that are ex-endosymbionts isolated from the protozoan Paramecium bursaria. Surprisingly, plaque-forming viruses on Syngen 2-3 lawns were often higher than on NC64A lawns from indigenous water samples. These differences led to the discovery of viruses that exclusively replicate in Syngen 2-3 cells, named Only Syngen (OSy) viruses. OSy-NE5, the prototype virus for the proposed new species, had a linear dsDNA genome of 327 kb with 44-nucleotide-long, incompletely base-paired, covalently closed hairpin ends. Each hairpin structure was followed by an identical 2,612 base-paired inverted sequence after which the DNA sequence diverged. OSy-NE5 encoded 357 predicted CDSs and 13 tRNAs. Interestingly, OSy-NE5 attached to and initiated infection in NC64A cells but infectious progeny viruses were not produced; thus OSy-NE5 replication in NC64A is blocked at some later stage of replication.

Chloroviruses

“…About 45 additional chlorovirus genomes have been sequenced and annotated, including members infecting all four host algae [93,94]. About 150 of the genes are common to all of the viruses, suggesting that they are important for virus replication.…”

Structure of the N-glycans from the chlorovirus NE-JV-1

“…Zoochlorellae are resistant to viruses in their symbiotic state, but the algae can grow independently from their hosts in the laboratory and thus one can study the virus-algal relationships. Viruses that infect zoochlorellae are classified into four groups depending on their hosts: NC64A viruses that infect Chlorella variabilis strain NC64A, Syn viruses that infect C. variabilis strain Syngen, SAG viruses that infect Chlorella heliozoae strain 3.83; and Pbi viruses that infect Micractinium conductrix strain Pbi (Quispe et al 2016;Van Etten et al 2010).…”

Biophysical Approaches to Solve the Structures of the Complex Glycan Shield of Chloroviruses

“…The structure of the glycans of the PBCV-1 major capsid protein Vp54 had remained elusive because the tools commonly used to study eukaryotic-like glycosylation patterns failed. This glycosylation pattern has been solved only recently (De Castro et al 2013) by combining three techniques, GC-MS, NMR and MALDI, and this same approach was effective when applied to other chlorovirus encoded glycoproteins (De Castro et al 2016;Quispe et al 2017;Speciale et al 2017). As a result, the structures of other chlorovirus N-glycans are now available, confirming that glycosylation is a trait common to these viruses.…”

“…Chloroviruses are divided in four groups depending on the host selectivity, and the genomes of 43 chloroviruses have been sequenced, assembled and annotated (Jeanniard et al 2013;Quispe et al 2017). Collectively, the viruses encode genes from 643 predicted protein families, and at least 17 of them are predicted to be involved in manipulating carbohydrates including (1) enzymes involved in making extracellular polysaccharides such as hyaluronan and chitin; (2) enzymes that make nucleotide sugars such as GDP-l-fucose and GDP-d-rhamnose; and (3) enzymes involved in the synthesis of glycans attached to the virus major capsid proteins (Van Etten et al 2010.…”

“…2); (4) the four glycoforms contain a dimethylated rhamnose as the capping residue of the main chain, a hyper-branched fucose residue, and two rhamnose residues with opposite absolute configurations; (5) the four glycoforms differ by the nonstoichiometric presence of two monosaccharides, l-arabinose and d-mannose; and (6) there is a core region (Fig. 2) strictly conserved in the N-glycans of all the chloroviruses studied to date (De Castro et al 2016;Quispe et al 2017;Speciale et al 2017). The glycan structures do not resemble any structure previously reported in the three domains of life.…”