Regulation of anti-CRISPR operons by structurally distinct families of Aca proteins

Abstract

CRISPR-Cas systems provide bacteria with adaptive immunity against bacteriophages and mobile genetic elements, driving an evolutionary arms race in which phages deploy anti-CRISPR (Acr) proteins. Acr proteins are often co-encoded in operons with anti-CRISPR-associated (Aca) proteins, which coordinate the regulation of acr gene expression. Here, we reveal the molecular basis of DNA binding that mediates transcriptional repression by two distinct Aca family members: Aca7 and Aca11. Crystal structures of Aca7 and Aca11 highlight conserved helix-turn-helix (HTH) motifs within α-helix bundles, providing a universal DNA-binding platform. Aca7 forms a symmetrical dimer to recognize a 19-bp inverted repeat (IR) within the acrIF11-aca7 operon. Strikingly, Aca11 binds 22-bp IRs in two distinct promoters, suggesting that Aca proteins can control multiple target operons. Mutagenesis and electrophoretic mobility shift assays (EMSAs) confirm that dimerization and sequence-specific IR recognition are essential for DNA binding. Despite mechanistic similarities, these and other Aca proteins exhibit notable differences. Structural comparisons across Aca families reveal that while monomer structures are generally similar with conserved HTH motifs, the structures of their dimeric functional units vary significantly. These structural differences might be essential for Aca proteins to bind to various promoters and regulate the expression of different Acr proteins.