engleski

A tryptophan ‘gate’ in the CRISPR-Cas3 nuclease controls ssDNA entry into the nuclease site in Escherichia coli

Cas3 is a ssDNA-targeting nuclease-helicase essential for class 1 prokaryotic CRISPR immunity systems. Cas3-DNA crystal structures show that ssDNA follows a pathway from helicase domains through to a HD-nuclease active site, requiring protein conformational flexibility during DNA translocation. In genetic studies we had noted that the efficacy of Cas3 in CRISPR immunity was drastically reduced when temperature was increased from 30 oC to 37 oC, by unknown mechanism. Here using E. coli Cas3 proteins we show that inhibition of nuclease activity at higher temperature corresponds with measurable changes in protein structure. This effect of temperature on Cas3 was alleviated by changing a single highly conserved tryptophan residue (Trp-406) into an alanine. The Cas3W406A protein is a hyperactive nuclease that functions independently from temperature and from the interference effector module Cascade. Trp-406 is situated at the interface of Cas3 HD and RecA1 domains that is important for maneuvering DNA into the nuclease active site. Molecular dynamics simulations based on the experimental data showed temperature- induced changes in Trp-406 positioning that either blocked or cleared the ssDNA pathway. We propose that Trp-406 forms a ‘gate’ for controlling Cas3 nuclease activity via access of ssDNA to the nuclease active site. The effect of temperature in these experiments may indicate allosteric control of Cas3 nuclease activity caused by changes in protein conformations.

helicase ; Cas3 ; CRISPR ; genome editing

nije evidentirano