Biological properties and genomic analysis of the fourth strain of molliviruses encoding a eukaryotic-like transmembrane transport protein

Giant viruses challenge the traditional definition of viruses due to their large virion sizes, intricate genomes, and enigmatic mechanisms of host modulation. So far, only three giant viruses belonging to the mollivirus group have been identified, resulting in a significant gap in our understanding of the biological properties of mollivirus. In this study, we present the isolation and in-depth characterization of the fourth strain of molliviruses, Mollivirus R33X, which was isolated from a soil sample in the Chinese subtropical zone. Our results demonstrate that the replication cycle of R33X spans approximately 24 h, yielding a burst size of about 266 viral particles per infected cell. R33X particles exhibit tolerance to acidic and high-salinity conditions yet show sensitivity to thermal variations. Its 645-kb genome, featuring inverted terminal repeats and comprising 557 genes, shares an average nucleotide identity of 95.8% with Mollivirussibericum, indicating a high degree of genetic conservation despite distant geographic isolation. Notably, we identify nineteen intron-containing genes in R33X, including a major facilitator superfamily transporter that is structurally homologous to eukaryotic counterparts. Structural modeling of viral protein complexes suggested that the transporter may facilitate the transport of small molecules, potentially altering host nutrient uptake during infection. Collectively, our work offers the most comprehensive biological analysis of mollivirus to date, highlighting the unique physiological and genetic features of mollivirus as well as its capacity to modulate host metabolism.