The Kinetic and Thermodynamic Aftermath of Horizontal Gene Transfer Governs Evolutionary Recovery

Shared host cells can serve as melting pots for viral genomes, giving many phylogenies a web-like appearance due to horizontal gene transfer. However, not all virus families exhibit web-like phylogenies. Microviruses form three distinct clades, represented by phi X174, G4, and alpha 3. Here, we investigate protein-based barriers to horizontal gene transfer between clades. We transferred gene G, which encodes a structural protein, between phi X174 and G4, and monitored the evolutionary recovery of the resulting chimeras. In both cases, particle assembly was the major barrier after gene transfer. The G4 phi XG chimera displayed a temperature-sensitive assembly defect that could easily be corrected through single mutations that promote productive assembly. Gene transfer in the other direction was more problematic. The initial phi XG4G chimera required an exogenous supply of both the phi X174 major spike G and DNA pilot H proteins. Elevated DNA pilot protein levels may be required to compensate for off-pathway reactions that may have become thermodynamically and/or kinetically favored when the foreign spike protein was present. After three targeted genetic selections, the foreign spike protein was productively integrated into the phi X174 background. The first adaption involved a global decrease in gene expression. This was followed by modifications affecting key protein-protein interactions that govern assembly. Finally, gene expression was re-elevated. Although the first selection suppresses nonproductive reactions, subsequent selections promote productive assembly and ultimately viability. However, viable chimeric strains exhibited reduced fitness compared with wild-type. This chimera's path to recovery may partially explain how unusual recombinant viruses could persist long enough to naturally emerge.