Mutual benefits from symbiotic coexistence between bipolar Euplotes cells and Francisella bacteria

Ciliates are common carriers of ecto- and endosymbiotic bacteria of which little is known about the molecular basis of the mutualistic relationships with the host. Working on interbreeding bipolar populations of the ciliate, Euplotes nobilii, members of these populations were found to stably host γ-proteobacteria of a new species of Francisella, F. adeliensis, in their cytoplasm. To inquire whether this coexistence entails mutual benefits, we analyzed the E. nobilii genome composition along with the genome composition of stably cultivated F. adeliensis colonies. The E. nobilii genome revealed no gene encoding methionine sulfoxide reductase of type A (MsrA), an enzyme which is essential for the antioxidant defense of every aerobic organism. In the F. adeliensis genome, we instead identified genes encoding two MsrA sequences endowed with a N-terminal signal peptide for the secretion into the host’s cytoplasm. And the effectiveness of this secretion was supported also by the identification of a complete gene set for the so-called Type VI Secretion System, a molecular machine that many Gram-negative bacteria use to transfer their proteins into target cells. E. nobilii cells thus appear to rely on the synthetic activity of F. adeliensis MsrA genes to face damage from oxidative stress imposed by the high (saturated) oxygen concentrations of the polar sea waters. On the other side, the bacterial genome lacks genes encoding enzymes involved in the biosynthetic pathways of cysteine, lysine, methionine, and threonine. F. adeliensis cells thus appear to rely on their host to obtain these four essential amino acids.