Prevalence and diversity of TAL effector-like proteins in fungal endosymbiotic spp.

Endofungal (formerly ) spp. rely on a type III secretion system to deliver mostly unidentified effector proteins when colonizing their host fungus, . The one known secreted effector family from consists of homologues of transcription activator-like (TAL) effectors, which are used by plant pathogenic and spp. to activate host genes that promote disease. These ' TAL-like (Btl)' proteins bind corresponding specific DNA sequences in a predictable manner, but their genomic target(s) and impact on transcription in the fungus are unknown. Recent phenotyping of Btl mutants of two strains revealed that the single Btl in one strain enhances fungal membrane stress tolerance, while others in a strain promote bacterial colonization of the fungus. The phenotypic diversity underscores the need to assess the sequence diversity and, given that sequence diversity translates to DNA targeting specificity, the functional diversity of Btl proteins. Using a dual approach to maximize capture of Btl protein sequences for our analysis, we sequenced and assembled nine spp. genomes using long-read PacBio technology and also mined available short-read Illumina fungal-bacterial metagenomes. We show that genes are present across diverse strains from Mucoromycota fungal hosts yet vary in sequences and predicted DNA binding specificity. Phylogenetic analysis revealed distinct clades of Btl proteins and suggested that might contain more species than previously recognized. Within our data set, Btl proteins were more conserved across strains than across , but there was also evidence of greater overall strain diversity within the latter clade. Overall, the results suggest that Btl proteins contribute to bacterial-fungal symbioses in myriad ways.