RNA-guided transcriptional repression by TIGR-Tas systems

Tandem interspaced guide RNA (TIGR)-TIGR-associated protein (Tas) systems constitute a diverse family of RNA-guided DNA-targeting proteins. Despite their widespread presence, their functional diversity has remained unexplored due to the lack of sequence conservation in their arrays, making them incompatible with existing annotation tools. Researchers have developed TIGRFinder, a motif-based pipeline, which has successfully identified 6,685 TIGR arrays from genomic and metagenomic data.

Phylogenetic and structural analysis of Tas proteins has unveiled clade-specific insertions in stem-loop binding Tas proteins. These insertions are found to co-vary with features of their corresponding tigRNAs. Cryo-electron microscopy structures of two stem-loop binding TasR ribonucleoprotein complexes have been determined, illustrating how these protein insertions facilitate extended tigRNA stems while preserving DNA binding through catalytically inactive RuvC domains.

Moreover, these catalytically dead TasR proteins, in conjunction with the non-nuclease-lacking TasA, have been shown to function as RNA-guided transcriptional repressors. This discovery establishes TIGR-Tas as a functionally versatile family of RNA-guided effectors. Comprehensive annotations of these systems are now available through TIGRSafari (https://tigr.bio), paving the way for further exploration and potential engineering applications.