Corynebacterium diphtheriae strains were analyzed using single strand conformational polymorphism (SSCP).
We found that only two strains carried mutations; these strains also showed the highest levels of the toxin pro-
duction (over 5120 VERO CD50/ml). Other strains were characterized either as high (640–5120 VERO
CD50/ml, 41 strains) or low (40-320 VERO CD50/ml, 38 strains) toxin producers. The nucleotide sequence
analysis revealed single T to C mutations at positions –54 and –184 within the –232 to +85 region of the tox
operon. The first mutation at position –184 was mapped outside of the tox promoter/operator, whereas the sec-
ond substitution at position –54 modified the 9-bp interrupted palindromic sequence of the tox promoter/oper-
ator from ATAATTAGG in the wild-type bacteriophage
to ACAATTAGG in strains with an enhanced level of the toxin production. The nucleotide sequence analysis of region from –76 to +681 of the diphtheria toxin
repressor (dtxR) gene from 15 strains of C. diphtheriae revealed two missense mutations resulting in amino acid
substitutions A147V and L214I in the C-terminal (the third) region of the DtxR protein. Seven of these strains
were identified as high toxin producers; four strains, as low toxin producers. In addition, one low-level toxin-
producing strain was shown to contain a missense mutation leading to the amino acid substitution I221T. Three
strains—two strains exhibiting the highest level of the toxin production and the nontoxigenic C7(–) strain—
contained no nucleotide substitutions. The ten strains belonging to the Sankt-Peterburg and Rossija epidemic
ribotypes, as well as the NCTC 13129 strain (the etiologic agent of the diphtheria epidemic outbreak in the East-
ern Europe), contained two mutations, A147V and L214I, in the C-terminal region of the diphtheria toxin
repressor (DtxR).