A new giant virus isolated from its natural host Saccamoeba sp. together with its virophage shows a narrow host range

Abstract

During last years, giant viruses have been found in some protists like Cafeteria and Acanthamoeba. The latter genus was often used as a bait to extract giant viruses from different habitats. Here, we present a natural host-parasite system isolated directly from the environment. A Saccamoeba sp. was isolated from the bark of a sycamore tree and a culture was established (Saccamoeba strain SL-5). Inside the amoebae, a Mimivirus-like endoparasite (isolate KSL-5) with a virion size of 430-450 nm was found via transmission electron microscopical investigations. The replication of the virus has a long lag phase of at least 12 h. After 24 h, an additional small icosahedral virus (about 50-60 nm) became visible within the virus factory. This satellite virus / virophage seems to interfere with the replication of KSL-5. The giant virus could be re-isolated (isolate KSL-5x) from the same tree using a virus-free strain of Saccamoeba. KSL-5x did not show any affiliation with a satellite virus / virophage but possessed a tail whereas a tail was found in only a few exemplars of KSL-5. All attempts to transfer KLS-5 and KSL-5x to other amoeba strains including Acanthamoeba sp. have failed, indicating a rather narrow host range, whereas both virus isolates appeared to be susceptible for the virophage. A preliminary phylogenetic analysis of the Mimivirus shows that KSL-5x is related to the Mimiviridae sensu stricto rather than to other giant viruses and most likely forms an independent branch basal to the Mimiviridae lineages A, B, and C. The closest relationship was detected to the newly described Tupanvirus, a caudate virus related to Mimiviridae. The preliminary analysis of the virophage revealed a "DNA polymerase type B, organellar and viral family protein [Trichomonas vaginalis G3]" indicating the virophage derived from a transposon of the Polinton family.