Guarani Virophage, a New Sputnik-Like Isolate From a Brazilian Lake

Abstract

Virophages are critical regulators of viral population dynamics and potential actors in the stability of the microbial networks. These small biological entities predate the replicative cycle of giant viruses, such as the members of the Mimiviridae family or their distant relatives, which produce within the cytoplasm of their host cells a viral factory harboring a complex biochemistry propitious to the growth of the smaller parasites. In this paper, we describe the isolation and the characterization of a new virophage, the eighth, that we named Guarani. We observed that Guarani exhibits a late replication cycle compared to its giant virus host. In addition, like all Sputnik strains, Guarani is able to infect the three lineages A, B and C of the Mimiviridae family, and affects the replication and the infectivity of its host virus. In terms of genetic content, Guarani has a 18,967 bp long double-stranded DNA genome encoding 22 predicted genes very similar to Sputnik genes, except for ORF19 and ORF12. The former is more related to Zamilon while the latter seems to be novel. The architecture of the Guarani genome is closely related to Sputnik and Zamilon strains, suggesting a common origin for all these virophages.

First evidence of host range expansion in virophages and its potential impact on giant viruses and host cells

“…Figure 5a shows that mutant Guarani decreases the replication of Tupanvirus DNA by approximately 3-fold. This low inhibition rate appears relatively similar to what has been found between the wild-type strain and APMV 24 . Experimental studies supported by bioinformatic analyses have shown that in contrast to their virus hosts, virophages become active during a late phase of giant virus infection 17,24,35 .…”

“…The Guarani genome consists of a circular dsDNA genome of 18,967 base pairs encoding 22 predicted genes very similar to Sputnik 24 . We found that the virophage isolated from APMV supernatant has no mutations in its genome compared to the original strain that we sequenced previously 24 . We therefore consider this strain to be the wild-type genotype of the virophage.…”

“…This probably raises questions regarding the virulence of the mutant genotype toward its new virus host, Tupanvirus. Indeed, it is known from our previous study that Guarani impaired the infectivity of its viral host APMV, resulting in a decrease in amoebae lysis 24 . Nevertheless, as this is the first time that we describe a host range expanding mutation in virophage, we expected the mutant genotype to have a distinct virulent profile toward its novel virus host than the wild-type strain toward APMV.…”

“…We used distant mimivirus relatives 21 Tupanvirus Deep Ocean and Tupanvirus Soda Lake as models to study the host range of virophages and their ability to replicate in amoebae infected with other mimiviruses than those belonging to the three lineages (A, B and C) of the family Mimiviridae 23 . Three virophages characterized in earlier studies were assayed with these giant viruses, including Guarani, Sputnik and Zamilon virophages 6,12,24 . These virophages were previously propagated in Acanthamoeba castellanii cells coinfected with mimiviruses and then purified.…”

“…We then analyzed whether host acquisition of Guarani was associated with a fitness trade-off with the prototype isolate of genus Mimivirus, Acanthamoeba polyphaga mimivirus (APMV), which has been used beforehand to propagate the wild-type genotype of Guarani 24,25 . Indeed, experimental evolution studies for other viruses have shown that increasing the virus fitness in one host could result in a fitness penalty in another host 1 .…”