Sibling Rivalry in Myxococcus xanthus Is Mediated by Kin Recognition and a Polyploid Prophage

Abstract

Myxobacteria form complex social communities that elicit multicellular behaviors. One such behavior is kin recognition, in which cells identify siblings via their polymorphic TraA cell surface receptor, to transiently fuse outer membranes and exchange their contents. In addition, outer membrane exchange (OME) regulates behaviors, such as inhibition of wild-type Myxococcus xanthus (DK1622) from swarming. Here we monitored the fate of motile cells and surprisingly found they were killed by nonmotile siblings. The kill phenotype required OME (i.e., was TraA dependent). The genetic basis of killing was traced to ancestral strains used to construct DK1622. Specifically, the kill phenotype mapped to a large "polyploid prophage," Mx alpha. Sensitive strains contained a 200-kb deletion that removed two of three Mx alpha units. To explain these results, we suggest that Mx alpha expresses a toxin-antitoxin cassette that uses the OME machinery of M. xanthus to transfer a toxin that makes the population "addicted" to Mx alpha. Thus, siblings that lost Mx alpha units (no immunity) are killed by cells that harbor the element. To test this, an Mx alpha-harboring laboratory strain was engineered (by traA allele swap) to recognize a closely related species, Myxococcus fulvus. As a result, M. fulvus, which lacks Mx alpha, was killed. These TraA-mediated antagonisms provide an explanation for how kin recognition specificity might have evolved in myxobacteria. That is, recognition specificity is determined by polymorphisms in traA, which we hypothesize were selected for because OME with non-kin leads to lethal outcomes. IMPORTANCEThe transition from single cell to multicellular life is considered a major evolutionary event. Myxobacteria have successfully made this transition. For example, in response to starvation, individual cells aggregate into multicellular fruiting bodies wherein cells differentiate into spores. To build fruits, cells need to recognize their siblings, and in part, this is mediated by the TraA cell surface receptor. Surprisingly, we report that TraA recognition can also involve sibling killing. We show that killing originates from a prophage-like element that has apparently hijacked the TraA system to deliver a toxin to kin. We hypothesize that this killing system has imposed selective pressures on kin recognition, which in turn has resulted in TraA polymorphisms and hence many different recognition groups.

Multifaceted Interfaces of Bacterial Competition

“…Motile cells of M. xanthus are killed when cultured with their nonmotile siblings. Killing is dependent upon the presence of TraA in the target motile cell and a polyploid prophage in the killer nonmotile sibling (130). Currently, the effector delivered by OME is not known, but it is likely produced from a toxin-antitoxin module encoded on the prophage (130).…”

Cell‐cell recognition and social networking in bacteria

“…TraA serves as the first layer of recognition for determining OME partners. A second layer of recognition further distinguishes true clonemates for cooperation (Dey et al, 2016;Vassallo et al, 2017). This second layer involves the exchange of polymorphic toxins that reside in the OM.…”

Self-identity reprogrammed by a single residue switch in a cell surface receptor of a social bacterium

“…Although traA is a greenbeard gene, OME also involves discrimination against kin because other loci impact OME. Our recent findings suggest that toxins encoded on a prophage are transferred by OME and partnering cells lacking cognate antitoxins are killed (17). Thus, TraA binding leads to self-recognition and the exchange of toxins further discriminates self from nonself.…”