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Thanks for your thoughtful comment, Paul. Especially interesting was your observation that the SaPIs could be considered to be parasites exploiting the phage. Indeed, they do decrease the phage burst size, presumably by being efficient at assembling the phage capsid proteins into the small SaPI capsids. (Also, a point that was not included in the post, the SaPIs encode one subunit of the phage terminase. It combines with a phage-encoded subunit to yield a terminase that selectively processes the SaPI DNA for packaging.) This 3-way interaction appears to be stable, which from my point of view seems to suggest that it works for all the members. Our language, which offers us words such as parasitic, symbiotic, and mutualistic, cannot convey the complexities of such relationships.

I also find myself thinking about the factors that came together to make this particular interaction work with these particular players. For example, this particular capsid protein can be assembled into a smaller capsid which can accommodate most SaPI genomes. In any event, it adds another dot to my imaginary dot plot of the different strategies used by nucleic acids to replicate and get around.


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