FIG 1.

Effect of stpA inactivation on natural transformation with CRISPR-Cas-targeted plasmid. (A) Schematic representation of natural transformation of E. coli for evaluating the CRISPR-Cas-mediated immunity against extracellular DNA. (B) Natural transformation was performed by spreading the mixture of the 24-h-incubated E. coli culture and plasmid onto LB plates containing 5% (wt/vol) agar supplemented with ampicillin (100 μg ml⁻¹). Levels of natural transformation with the CRISPR-Cas-targeted plasmid pCR1 (pT) or nontargeted plasmid pDsRED (pNT) were measured in Δhns, ΔstpA, Δhns ΔstpA, and WT strains. Cell viability of each transformant was measured (see Fig. S9 in the supplemental material). Data are shown as means ± standard deviations (n = 3). Statistical significance was determined using a two-tailed Student's t test (**, P ≤ 0.01; ***, P ≤ 0.005).