Architectural principles for Hfq/Crc-mediated regulation of gene expression

SUMMARY The global regulator Hfq facilitates the action of regulatory RNAs in post-transcription gene regulation in many Gram-negative bacteria. In Pseudomonas aeruginosa , Hfq, in conjunction with the catabolite repression protein Crc, was shown to form a complex that directly inhibits translation of target transcripts during carbon catabolite repression. Here, we describe and validate high-resolution cryo-EM structures of the cooperative assembly of Hfq and Crc bound to a translation initiation site. The core assembly is formed through interactions of two cognate RNAs, two Hfq hexamers and a Crc pair. Additional Crc protomers can be recruited to form higher-order assemblies with demonstrated in vivo activity. The structures indicate a distinctive RNA conformation and a pattern of repeating motifs that confer regulatory function. This study not only reveals for the first time how Hfq cooperates with a partner protein to regulate translation but also provides a novel structural basis to explain how an RNA code can guide global regulators to interact cooperatively and regulate many different RNA targets.