A widespread family of phage-inducible chromosomal islands only steals bacteriophage tails to spread in nature

Phage satellites interfere with helper phage packaging through the production of small-capsids, where only satellites can be packaged. So far, in all the analysed systems, the satellite-sized capsids are composed of phage proteins. Here we report the first demonstration that a family of phage-inducible chromosomal island (PICIs), a type of satellites, encodes all the proteins required for both the production of the small-sized capsids and the exclusive packaging of the PICIs into these capsids. Therefore, this new family, that we have named cf-PICIs (capsid forming PICIs), only requires phage tails to generate infective PICI particles. Remarkably, the representative cf-PICI reproduces without cost for their helper phages, suggesting that the relationship between these elements is not parasitic but commensalistic. Finally, our phylogenomic studies indicate that cf-PICIs are present both in Gram-positive and Gram-negative bacteria and have evolved at least three times independently to spread widely into the satellite universe.