Conformational differences in bacterial ribosomal RNAs in non-denaturing conditions

THE conservative nature of bacterial ribosomal RNA (rRNA) is clear from the narrow range of its guanosine–cytosine (GC) content compared with the base composition of whole cell DNA. The majority of bacterial DNAs contain from 35 to 75 % GC, whereas the GC content of rRNA is restricted to a narrow range of 50–53% (ref. 1). Indeed, the ribosomal DNA (rDNA) of many bacterial species can be isolated from sheared preparations of whole cell DNA solely on the basis of its GC content. Furthermore, nucleic acid hybridisation has indicated considerable homology between the rRNAs of diverse bacterial species 2–4 and helped provide phylogenetic support for taxo-nomic relationships. In an investigation of 22 different bacteria Pace and Campbell found that organisms whose rRNA showed closer homology to Escherichia coli rRNA, showed less rRNA homology to Bacillus stearothermophilus rRNA and vice versa 2 . Both sedimentation and gel electrophoresis studies in non-denaturing conditions have shown very small, if any, migratory differences between either the 23S or 16S rRNA species of different bacterial species 5,6 . By using a modified buffer system coupled with double labelling, we have found that both the large and small species of rRNA from Streptococcus faecalis, B. subtilis , and E. coli can be all distinguished on the basis of significant differences in their mobility in both composite polyacrylamide–Agarose gels and polyacrylamide gels.