Characterization of the diphtheria tox transcript in Corynebacterium diphtheriae and Escherichia coli

Transcription of the tox gene in lysogenic Corynebacterium diphtheriae strains C7(beta tox+), C7 (gamma tox) and the hypertoxigenic PW8 (omega tox+) was analyzed and compared with transcription of the C. diphtheriae tox gene in the recombinant strain Escherichia coli (pDT201). In all cases S1 nuclease mapping localized the 5' terminus of the tox mRNA to a site 8 or 9 base pairs (bp) downstream of a region similar to the -10 consensus sequence of E. coli promoters. In C. diphtheriae the tox transcript was observed only in strains that were grown under iron-limiting conditions; in the presence of excess iron, transcription beyond bp 38 of the tox coding region was not observed. In contrast, in E. coli(pDT201) tox was expressed at equivalent levels in both iron-depleted and iron-supplemented media. The DNA insertion in the tox gene of the nontoxigenic corynephage gamma was found to occur at bp 54 of the tox coding region. The insertion event resulted in the duplication of a 7-bp target sequence, and the ends of the insert were found to constitute an imperfect inverted repeat of approximately 26 bp. Transcription from the tox promoter in C7(gamma tox) was found to initiate at the same nucleotides as in C7(beta tox+), PW8, and E. coli(pDT201) and remained sensitive to iron inhibition. These observations are discussed in relation to the mechanism of iron-mediated regulation of the tox gene.