Restriction of frog virus 3 polypeptide synthesis to immediate early and delayed early species by supraoptimal temperatures

Recombinant Ranaviruses for Studying Evolution of Host-Pathogen Interactions in Ectothermic Vertebrates

Ranaviruses (Iridoviridae) are large DNA viruses that are causing emerging infectious diseases at an alarming rate in both wild and captive cold blood vertebrate species all over the world. Although the general biology of these viruses that presents some similarities with poxvirus is characterized, many aspects of their replication cycles, host cell interactions and evolution still remain largely unclear, especially in vivo. Over several years, strategies to generate site-specific ranavirus recombinant, either expressing fluorescent reporter genes or deficient for particular viral genes, have been developed. We review here these strategies, the main ranavirus recombinants characterized and their usefulness for in vitro and in vivo studies.

ISOLATION OF A RANAVIRUS FROM A GECKO (UROPLATUS FIMBRIATUS)

A virus was isolated from the liver and stomach of a leaf-tailed gecko (Uroplatus fimbriatus) with granulomatous lesions on the tongue and hepatitis. The virus was identified as an iridovirus on the basis of morphology by electron microscopy, restriction endonuclease assay, and sequencing of a large portion of the major capsid protein gene. Comparative analysis revealed that this isolate is related to frog virus 3, the type species of the genus Ranavirus.

Organization of RNA transcripts from a 7.8-kb region of the frog virus 3 genome

The detailed organization of the RNAs transcribed from a region of the FV 3 genome (Sa/I-F fragment and adjacent sequences) has been determined. The information was derived from the cell-free translation of hybrid-selected RNA to locate the genes encoding specific polypeptides, RNA filter hybridization to size the transcripts, and S1 nuclease mapping to locate the 5'- and 3'-ends of the RNAs on the genome. Three genes are contiguous and are transcribed from the same strand: two immediate early genes encoding transcripts of about 1.3 kb that directed the in vitro synthesis of 42K and 46K polypeptides, separated by the late gene encoding the major capsid protein (48K). At an advanced stage in infection, transcripts derived from the immediate early genes are also present. A set of RNAs with different 5'-ends ranging from 1.7 to 0.58 kb is produced from the p46 gene region whereas RNAs, 0.98 and 0.6 kb in size, complementary to the 5'-end of the p42 message, are synthesized. This gene cluster is located between two genes transcribed in the opposite direction from the rightward-reading strand: a late gene whose message is 0.5 kb in size and encodes a 15K polypeptide and a gene transcribed at immediate early and late times of infection which encodes a protein of 70 kDa. The 5'-end of the late RNA maps downstream of the 5'-end of the early one, their sizes being 1.85 and 2 kb, respectively, but both of them can be translated in vitro into a 70K polypeptide. These observations suggest that transcription is not regulated by the organization of the genes; they suggest rather that specific DNA sequences are responsible for the promotion of immediate early and late transcriptions.

Molecular cloning and physical and translational mapping of the frog virus 3 genome

A library of cloned fragments representing nearly the entire frog virus 3 (FV 3) genome (99.65%) has been constituted. Individual plasmid recombinants, labeled by nick-translation, were hybridized to Southern blots of genomic FV 3 DNA fragments obtained with XbaI, HindIII, SmaI, and SalI. From these results physical maps were generated and the distribution of restriction sites in the genome was established by double digestion of the fragments. A preliminary translational map was likewise developed. The viral messages were selected by hybridization to the recombinant DNAs immobilized on nitrocellulose filters and were translated in the reticulocyte cell-free system. About 30 polypeptides were detected among the translation products of RNA synthesized in the presence of cycloheximide. It appears that these genes are not clustered but in several cases more than one polypeptide is encoded by a given fragment. The 15 new polypeptide obtained by translation of late mRNAs derive from genes located on one-half of the genome.

Thermosensitivity of frog virus 3 genome expression: defect in early transcription

The influence of temperature on the transcription of the frog virus 3 genome was studied in CHO cells infected both at 29 and at 37 degrees, the nonpermissive temperature for virus multiplication. It was definitely established that late genes were not transcribed at 37 degrees. Although immediate early genes were expressed at 37 degrees, their transcription was altered but there was no sequestration of mRNAs in the nucleus which could impair their translation; these viral mRNAs were also efficiently translated in vitro. These results indicate that an immediate early viral protein involved in the transcription of delayed early genes is likely to be thermosensitive. Furthermore, one event taking place at the very beginning of the infection, possibly related to the activity of a viral structural component, facilitates the transcription of immediate early genes at 29 degrees and this step is partially impaired at 37 degrees.