First-ever observation of virus latching onto another virus to infect its host

The cells of organisms such as bacteria or plants may become infected with viruses, know as satellites, which need the genetic material of other viruses, known as helpers, to replicate and spread within their host’s cells. While this process occurs, the two types of viruses need to be in close contact with each other. However, until now there was no known evidence of a satellite virus attaching to its helper virus, nor had scientists ever seen any type of attachments between other types of viruses.

This recent work describes the first-ever observations made of a satellite virus systematically latching on to its helper virus. Published in the Journal of the International Society of Microbial Ecology (Nature group), the study was coordinated by Ivan Erill, bioinformatics researcher at the UMBC and at the Department of Information and Communications Engineering, UAB. It also included the involvement of Júlia López-Pérez, pre-doctoral researcher from the Department of Genetics and Microbiology at the UAB.

The discovery was made in bacteriophages (viruses infecting bacterial cells) from the soil bacterium Streptomyces scabiei. Researchers discovered that the satellite virus latched on to the “neck” of its helper virus, close to the area where the viral capsid is connected to the tail. “This is the first case in which we identify a virus attaching itself to another virus”, explains Ivan Erill.

In the transmission electron microscopy (TEM) images observed by Tagide deCarvalho, researcher at the UMBC and first author of the study, 80% of helper bacteriophages (40 out of 50) contained a viral satellite attached to their neck. Others showed signs of having had one on their neck, “with bite mark-like signs”, researchers say.

Why this close relationship between the two?

Analysis of the genome of both bacteriophages and the host cell has provided a plausible explanation for this close relationship. Most satellite viruses have genes that allow them to integrate into the host cell's genetic material and remain dormant, waiting for a helper virus to enter and begin to reproduce. The host cell then copies the DNA of the satellite virus along with its own when dividing. However, the bacteriophage discovered does not have any integration genes.

“That is why it makes complete sense for the viral satellite to latch onto the helper virus”, points out Ivan Erill. “Our hypothesis is that since it cannot integrate into the DNA of the host cell, it needs to be as close as possible to the helper virus in order to enter at the same time and survive. And what better way to guarantee success than to latch on to the neck of its helper? We observed that it had developed a short appendage with the aim of better latching on to the helper virus”, he adds.

The new satellite bacteriophage and its helper have been named, respectively, MiniFlayer and MindFlayer by the undergraduate students participating in the study. Both viruses must have co-evolved for at least 100 million years, according to bioinformatics studies. The satellite would have been during all this time tuning and optimising its genome to align it with its helper’s.

The next step for researchers will be to try to find out how the satellite attaches and how common this phenomenon is. "It is possible that in the past it was assumed that various satellites such as the one identified today were simply contaminants found in the samples of their helpers. There could therefore be many more cases of this type of relationship waiting to be discovered", concludes Ivan Erill.

The UAB researcher and his collaborators carried out the study as part of a research programme to identify new bacteriophages for use in antibacterial therapies. The genetic competition between satellite bacteriophages and their helpers, as illustrated by this discovery, could be used to explore new ways of developing antiviral drugs.

Article: deCarvalho, T.; Mascolo, E.; Caruso, S. M., et al. «Simultaneous entry as an adaptation to virulence in a novel satellite-helper system infecting Streptomyces species». ISME J (2023). https://doi.org/10.1038/s41396-023-01548-0.