Envelope stress is a trigger of CRISPR RNA-mediated DNA silencing in Escherichia coli.

A widespread feature in the genomes of most bacteria and archaea is an array of clustered, regularly interspaced short palindromic repeats (CRISPRs) that, together with a group of CRISPR-associated (Cas) proteins, mediate immunity against invasive nucleic acids such as plasmids and viruses. Here, the CRISPR-Cas system was activated in cells expressing a plasmid-encoded protein that was targeted to the twin-arginine translocation (Tat) pathway. Expression of this Tat substrate resulted in upregulation of the Cas enzymes and subsequent silencing of the encoding plasmid in a manner that required the BaeSR two-component regulatory system, which is known to respond to extracytoplasmic stress. Furthermore, we confirm that the CasCDE enzymes form a stable ternary complex and appear to function as the catalytic core of the Cas system to process CRISPR RNA into its mature form. Taken together, our results indicate that the CRISPR-Cas system targets DNA directly as part of a defence mechanism in bacteria that is overlapping with but not limited to phage infection.

Exploration of twin-arginine translocation for expression and purification of correctly folded proteins in Escherichia coli.

Historically, the general secretory (Sec) pathway of Gram-negative bacteria has served as the primary route by which heterologous proteins are delivered to the periplasm in numerous expression and engineering applications. Here we have systematically examined the twin-arginine translocation (Tat) pathway as an alternative, and possibly advantageous, secretion pathway for heterologous proteins. Overall, we found that: (i) export efficiency and periplasmic yield of a model substrate were affected by the composition of the Tat signal peptide, (ii) Tat substrates were correctly processed at their N-termini upon reaching the periplasm and (iii) proteins fused to maltose-binding protein (MBP) were reliably exported by the Tat system, but only when correctly folded; aberrantly folded MBP fusions were excluded by the Tat pathway's folding quality control feature. We also observed that Tat export yield was comparable to Sec for relatively small, well-folded proteins, higher relative to Sec for proteins that required cytoplasmic folding, and lower relative to Sec for larger, soluble fusion proteins. Interestingly, the specific activity of material purified from the periplasm was higher for certain Tat substrates relative to their Sec counterparts, suggesting that Tat expression can give rise to relatively pure and highly active proteins in one step.

An essential role for the DnaK molecular chaperone in stabilizing over-expressed substrate proteins of the bacterial twin-arginine translocation pathway.

All secreted proteins in Escherichia coli must be maintained in an export-competent state before translocation across the inner membrane. In the case of the Sec pathway, this function is carried out by the dedicated SecB chaperone and the general chaperones DnaK-DnaJ-GrpE and GroEL-GroES, whose job collectively is to render substrate proteins partially or entirely unfolded before engagement of the translocon. To determine whether these or other general molecular chaperones are similarly involved in the translocation of folded proteins through the twin-arginine translocation (Tat) system, we screened a collection of E. coli mutant strains for their ability to transport a green fluorescent protein (GFP) reporter through the Tat pathway. We found that the molecular chaperone DnaK was essential for cytoplasmic stability of GFP bearing an N-terminal Tat signal peptide, as well as for numerous other recombinantly expressed endogenous and heterologous Tat substrates. Interestingly, the stability conferred by DnaK did not require a fully functional Tat signal as substrates bearing translocation defective twin lysine substitutions in the consensus Tat motif were equally unstable in the absence of DnaK. These findings were corroborated by crosslinking experiments that revealed an in vivo association between DnaK and a truncated version of the Tat substrate trimethylamine N-oxide reductase (TorA502) bearing an RR or a KK signal peptide. Since TorA502 lacks nine molybdo-cofactor ligands essential for cofactor attachment, the involvement of DnaK is apparently independent of cofactor acquisition. Finally, we show that the stabilizing effects of DnaK can be exploited to increase the expression and translocation of Tat substrates under conditions where the substrate production level exceeds the capacity of the Tat translocase. This latter observation is expected to have important consequences for the use of the Tat system in biotechnology applications where high levels of periplasmic expression are desirable.