Point mutations sites in tox promoter/operator and diphtheria toxin repressor (DtxR) gene associated with the level of toxin production by Corynebacterium diphtheriae strains isolated in Belarus

Abstract

DNA fragments 129 bp in length containing promoter region of the tox gene from 81 toxigenic strains Corynebacterium diphtheriae were analyzed using the SSCP (single strand conformational polymorphism). We found that only two strains had mutations; the strains also had highest levels of toxin production (over 5120 Vero CD50/ml). Other strains were characterized either as high-level toxin-producing (640-5120 Vero CD50/ml, 41 strains) or low-level toxin-producing (40-320 Vero CD50/ml, 38 strains). Nucleotide sequence analysis revealed single T to C mutations at positions -54 and -184 within -232 - +85 region of tox operon. The first mutation at the -184 position was mapped outside the tox promoter/operator, whereas the second substitution at the -54 position modified the 9-base-pair interrupted palindromic sequence of the tox promoter/operator from ATAATTAGG in the wild-type bacteriophage (to ACAATTAGG in strains with enhanced level of toxin production. Nucleotide sequence analysis of -76 - +681 region of diphtheria toxin repressor (dtxR) gene from 15 strains of C. diphtheriae revealed two missense mutations resulting in amino acid substitutions A 147 V; and L 214 I in the C-terminal region of the DtxR protein. Seven of these strains were identified as high-level toxin-producing and 4 strains, as low-level toxin-producing. In addition, one low-level toxin-producing strain was shown to contain a missense mutation leading to amino acid substitution I 221 T. Three strains, including two highest-level toxin producing strains contained no nucleotide substitutions, as well as the C7(-) strain. The 10 strains belonging to the Sankt-Peterburg and Rossija epidemic ribotypes as well as NCTC 13129 strain (etiologic agent of the diphtheria epidemic outbreak in the Eastern Europe) was shown to contain two mutations A 147 V and L 214 I in the C-terminal region of the DtxR protein.