Abstract
Ribosomal ribonucleic acid (rRNA) genes were isolated from a PstI digest of Pseudomonas aeruginosa chromosomal deoxyribonucleic acid (DNA), cloned in Escherichia coli, and used as probes for conserved gene sequences. Recombinant plasmid pHF1 contained an 8,800-base pair insertion containing 5S, 16S, and 23S rRNA genes. We constructed subclones of pHF1 containing parts of the 16S and 23S rRNA genes (pHF1.1) and parts of the 23S and 5S rRNA genes (pHF1.2). DNA-DNA hybridization experiments in which we used filter-bound chromosomal DNA from various bacteria and 35S-labeled plasmid rRNA genes (rDNA) indicated that the homology values reflected the actual phylogenetic distances to P. aeruginosa. Compared with oligonucleotide sequence analysis of 16S rRNA, a good correlation was found between DNA-rDNA homology values and SAB (similarity coefficient of 16S rRNAs) values above 0.4. The use of rDNA instead of rRNA in hybridization experiments offers several advantages; e.g., rDNA can easily be labeled in vitro, and the degree of relatedness can be expressed in terms of percent homolgy and does not have to be determined by laborious measurement of thermal stability, as in the case of rRNA.
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