Mutant characterization. (A) Southern blot analysis of HindIII-digested genomic DNA from wild-type (JIR8094) (a), prdB mutant (LB-CD4) (b), and grdA mutant (LB-CD12) (c) strains with an intron probe. (B) Radiolabeling (75Se) of C. difficile wild-type and mutant strains. Wild-type (JIR8094), grdA mutant (LB-CD12), and prdB mutant (LB-CD4) strains were grown in TY medium with 75Se (4 μCi) for 24 h, harvested, and lysed by sonication. The extracts were subjected to SDS-15% polyacrylamide gel electrophoresis.