A TAL effector-like protein of an endofungal bacterium increases the stress tolerance and alters the transcriptome of the host

Symbioses of bacteria with fungi have only recently been described and are poorly understood. In the symbiosis of Mycetohabitans (formerly Burkholderia) rhizoxinica with the fungus Rhizopus micro-sporus, bacterial type III (T3) secretion is known to be essential. Proteins resembling T3-secreted transcription activator-like (TAL) effectors of plant pathogenic bacteria are encoded in the three se-quenced Mycetohabitans spp. genomes. TAL effectors nuclear -localize in plants, where they bind and activate genes important in disease. The Burkholderia TAL-like (Btl) proteins bind DNA but lack the N-and C-terminal regions, in which TAL effectors harbor their T3 and nuclear localization signals, and activation domain. We characterized a Btl protein, Btl19-13, and found that, despite the structural differences, it can be T3-secreted and can nuclear -localize. A btl19-13 gene knockout did not prevent the bacterium from infecting the fungus, but the fungus became less tolerant to cell membrane stress. Btl19-13 did not alter transcription in a plant -based reporter assay, but 15 R. microsporus genes were differen-tially expressed in comparisons both of the fungus infected with the wild-type bacterium vs. the mutant and with the mutant vs. a com-plemented strain. Southern blotting revealed btl genes in 14 diverse Mycetohabitans isolates. However, banding patterns and available sequences suggest variation, and the btl19-13 phenotype could not be rescued by a btl gene from a different strain. Our findings sup-port the conclusion that Btl proteins are effectors that act on host DNA and play important but varied or possibly host genotype -specific roles in the M. rhizoxinica-R. microsporus symbiosis.