Comparison of the alpha-toxin genes of Clostridium perfringens type A and C strains: evidence for extragenic regulation of transcription.

The Clostridium perfringens plc gene encoding phospholipase C (alpha-toxin) was cloned from type C NCIB 10662, a strain which produces low levels of phospholipase C activity. The nucleotide sequence of a cloned 3.1-kb HindIII fragment was determined. The same fragment was also cloned from type A NCTC 8237, a phospholipase C-overproducing strain. In this case, an open reading frame (ORF2) truncated in the previously cloned 2-kb fragment was also sequenced. Comparison of the nucleotide sequence between the 3.1-kb fragments of the two type strains shows some differences both in the plc gene and in ORF2. However, when the 3.1-kb fragment was cloned into plasmid pUC19 and expressed in Escherichia coli, the plc genes from both type strains were similarly expressed and the toxins produced showed similar levels of activity. Northern blot analysis revealed that the type A strain produced 16 to 23 times more plc mRNA than the type C strain. These results indicate that in C. perfringens the two plc genes are transcribed at different rates, probably because of a difference in a locus lying outside of the cloned fragments. Gel retardation analysis showed that the type A strain possessed two different proteins that bound different regions of the plc gene. However, one of these proteins, which binds within the plc coding region, was not found in the type C strain, suggesting that it plays a role in the regulation of the plc gene expression.