Fig. 7. A revised model for eCIS function and ecology.

eCIS is encoded by operons that are enriched in environmental microbes. These operons are regulated by adjacent genes such as RfaH⁷⁸ or genes that carry GerE or BTAD domains. The regulators are occasionally activated by kinases and are activated/repressed by yet unknown signals. eCIS-associated toxins (EATs) tend to be encoded in the 3′ end of the operons. EATs often carry DUF4157 domains in their N-terminus or found adjacent to genes with this domain. The eCIS particle is assembled with the toxins inside the tube¹⁵ and is released following cell lysis¹³. The particles carry tail fibers that mediate attachment to environmental eukaryotic or prokaryotic target cells. The tail fibers attach, contract, and release the EATs inside the target cells, mostly leading to its death or growth arrest. The EATs function intracellularly in diverse ways, enzymatically acting on different cellular components that were identified in the current and in previous works. Question marks denote aspects of eCIS function that are predicted based on our results yet have not been experimentally confirmed.