Dealing with the Evolutionary Downside of CRISPR Immunity: Bacteria and Beneficial Plasmids
(A) Summary of the different mutations found in this study and their proportions. (B) Distribution of mutations within the CRISPR-Cas locus. S. epidermidis RP62a harbors a CRISPR-Cas system containing four repeats (white boxes), three spacers (colored, numbered boxes) and nine cas/csm genes. Mutations found in CRISPR escapers include deletions in the repeat-spacer region (brackets), transposon insertions (red arrowheads; top, direct insertion; bottom, inverted) and single nucleotide deletions or substitutions (asterisks). Arrows indicate primers used to analyze transconjugants. (C) PCR analysis of the CRISPR array of transconjugants using primers L50/L6. Deletion of 1, 2 and 3 spacers observed in escapers R23, R10 and R2, respectively, is shown. M, DNA marker. wt, amplification using wild-type template DNA. (D) PCR analysis of the cas gene region of escapers using primers L23/L106. IS256 transposon insertions into csm5, csm6 and cas6 observed in escapers R60, B15 and R36, respectively, are shown. M, DNA marker. wt, amplification using wild-type template DNA.