Detection of a group II intron without an open reading frame in the alpha-toxin gene of Clostridium perfringens isolated from a broiler chicken.

A DNA insertion of 834 bp, designated CPF-G2Im, was identified within the alpha toxin gene (cpa) of Clostridium perfringens strain CPBC16ML, isolated from a broiler chicken. Sequence analysis of CPF-G2Im indicated that it was integrated 340 nucleotides downstream of the start codon of cpa. However, the insertion did not abolish the phospholipase C and hemolytic activities of CPBC16ML. To investigate the expression of its alpha toxin, the intact copy of cpa was cloned into an expression vector and transformed into Escherichia coli M15 cells. Immunoblotting analysis showed that the protein expressed from the transformant as well as in the culture supernatant of C. perfringens strain CPBC16ML had the expected molecular weight detected in reference strains of C. perfringens. Northern hybridization and reverse transcriptase PCR (RT-PCR) analysis revealed that the entire CPF-G2Im insertion was completely spliced from the cpa precursor mRNA transcripts. The sequence of the insertion fragment has 95% and 97% identity to two noncoding regions corresponding to sequences that flank a predicted group II RT gene present in the pCPF4969 plasmid of C. perfringens. However, an RT was not encoded by the CPF-G2Im fragment. Based on the secondary structure prediction analysis, CPF-G2Im revealed typical features of group II introns. The present study shows that CPF-G2Im is capable of splicing in both C. perfringens and E. coli. To our knowledge, this is the first report that a group II intron without an open reading frame (ORF) is located in the cpa ORF of C. perfringens.