Doi, Roy H. (Syracuse University, Syracuse, N.Y.), and Richard T. Igarashi. Conservation of ribosomal and messenger ribonucleic acid cistrons in Bacillus species. J. Bacteriol. 90:384-390. 1965.-Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) fractions from various Bacillus species were tested for interspecies DNA-RNA hybrid formation. DNA preparations from B. subtilis, B. cereus, B. megaterium, B. stearothermophilus, and B. macerans, whose base composition varied from 33 to 50% guanylate + cytidylate content, were used in the hybrid annealing mixtures with pulse-labeled RNA from sporulating cells and from log-phase cells of B. subtilis and B. cereus. Efficient hybridization in these cases was obtained only in homologous annealing situations. When heterologous DNA and RNA preparations were tested for hybrid formation, only 1 to 6% of the homologous hybridization was obtained. Although the efficiency of hybrid formation was low, the results were reproducible. No difference in efficiency of hybrid formation was observed between the messenger RNA from sporulating cells and that from log-phase cells. When B. subtilis ribosomal RNA was placed in annealing mixtures with heterologous DNA, 47.5 to 62.9% of the homologous hybridization was obtained. These results suggest that a small number of identical sequences are present among the Bacillus species. Furthermore, the ribosomal RNA cistrons appear to be more highly conserved relative to other genetic sequences.