engleski

Cas3 stimulates runaway replication of a ColE1 plasmid in Escherichia coli and antagonises RNaseHI

Cas3 nuclease-helicase is part of CRISPR immunity systems in many bacteria and archaea. In type I CRISPR, Cas3 nuclease degrades invader DNA that has been base-paired to crRNA as an R-loop within a “Cascade” complex. An R-loop is a DNA-RNA hybrid that includes a displaced single strand DNA loop. Purified Cas3 from E. coli and the archaeon M. thermautrophicus can process R-loops without DNA/RNA sequence specificity and without Cascade. This has potential to affect other aspects of microbial biology that involve R-loops. Regulatory RNAs and host cell proteins modulate replication of ColE1 plasmids (e.g. pUC) from R-loop primers. We observed that Cas3 could over-ride endogenous control of a ColE1 replicon, stimulating deregulated (“runaway”) replication and resulting in much higher plasmid yields. This effect was absent when using helicase defective Cas3 (Cas3K320L) or a non-ColE1 plasmid, and was dependent on RNaseH. Cas3 also promoted plasmid multimer (concatemer) formation, a phenotype consistent with deregulated ColE1 replication and typical of cells lacking RNaseH. These effects of Cas3 on ColE1 plasmids are inconsistent with it unwinding R-loops in vivo, at least in this assay. We discuss how Cas3 might target other RNA molecules in vivo, as a potential explanation for deregulated plasmid replication, unless it is kept in check by Cascade or host R-loop regulators RecG and RNaseH.

CRISPR ; ColE1 ; cas3 ; RNaseHI ; R-loop ; recG

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