|Ladely, Scott -|
Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2010
Publication Date: May 17, 2010
Citation: Meinersmann, R.J., Ladely, S.R., Lindsey, R.L. 2010. Ribosomal operon intergenic sequence region (ISR) heterogeneity in Campylobacter coli and Campylobacter jejuni. Letters in Applied Microbiology. 51(5):539-545. Interpretive Summary: One of the unresolved questions in the Theory of Evolution is how new functions can come into being. The leading hypothesis is that pre-existing genes will duplicate so that there are two or more copies within one cell. Then one copy of the gene continues to fulfill the original function and the second copy evolves to efficiently fulfill a new function. The problem with this hypothesis has been that a process known as ‘concerted evolution’ usually occurs that tends to lock duplicated genes into maintaining the same form. The question is how duplicated genes escape from concerted evolution. Duplicated genes can be advantageous to cells when the function they code for is needed at higher concentrations that can be provided by expression from the multiple copies. Such is the case for ribosomes that are required for protein synthesis and most cells have multiple copies of the ribosomal genes that are maintained in very close similarity. Campylobacter is a genus of bacteria that are common food-borne pathogens. These bacteria have three copies of the ribosome genes. We have observed that in most individual Campylobacter cells all three copies are very similar if not identical. However, we found one strain in which two copies of the gene were characteristic of the structure seen in Campylobacter coli and one copy was characteristic of Campylobacter jejuni. This change could only have been brought about by genetic exchange between the two species and then only maintained if there was a defect in the mechanism for concerted evolution. We have observed in a previous study that concerted evolution of the ribosome genes is very rapid, all three copies of the gene maintain the same form after overnight growth. Thus, we believe that we have identified a concerted evolution defective strain and that this could provide a background for duplicated genes to escape concerted evolution and diverge to new functions.
Technical Abstract: Campylobacter jejuni and Campylobacter coli are closely related species that can not be distinguished by their 16S or 23S rRNA gene sequences. However, the intergenic sequence region (ISR) that is between the 16S and 23S genes is markedly different and characteristic for each species. A peculiarity of the genomic sequence for C. coli RM2228 (Genbank # AAFL01000000) is that two of the rRNA operons have an IGS that is characteristic of C. coli and the third operon is characteristic of C. jejuni. Primers for PCR were designed such that the characteristic C. coli product would be 327 base-pairs (bp) and the C. jejuni product would be 166 bp. A strain with the heterogeneity like that in C. coli RM2228 should yield products of both sizes. To determine the extent of the congruence of the IGS with the C. jejuni/coli species, DNA from a panel of randomly collected C. coli (n=133) and C. jejuni (n=134) isolates were tested by PCR with the primers. All of the isolates yielded products of the predicted size for the species and none yielded fragments of both predicted sizes. To verify the Genbank data for C. coli RM2228, each ribosomal operon from the isolate was individually amplified by PCR and tested with the IGS primer pair. Products of both sizes were produced from the correct operon as predicted from the genomic data. Thus, the cross species heterogeneity of the IGS seen in C. coli RM2228 is infrequent and that strain is extraordinary for its ribosomal operon structure. This may be a consequence of defective concerted evolution of the duplicated ribosomal operons in the affected strain.