sentence: Although a null sigD mutation does not affect sporulation under these conditions ( comparing the 630 Δ erm and 630 Δ erm sigD vector control strains ) ( Fig. 6A and F ) ( 39 , 49 ) , we chose to investigate the sigD mutant because c-di-GMP directly represses sigD transcription , inhibiting C. difficile motility and toxin production ( 13 , 15 , 50 ) .