Revertants of a streptomycin-resistant, oligosporogenous mutant ofBacillus subtilis

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Abstract
Revertants of a streptomycin-resistant (StrR), oligosporogenous (Spo-) mutant ofBacillus subtilis were selected for the ability to sporulate. The revertants obtained fell into two phenotypic classes: StrS Spo+ (streptomycin-sensitive, sporeforming), which arose by reversion of the streptomycin resistance mutations of the parent strain; and StrR Spo+, which arose by the acquisition of additional mutations, some of which were shown to affect ribosomal proteins. Alterations of ribosomal proteins S4 and S16 in the 30S subunit and L18 in the 50S subunit were detected in StrR Spo+ revertants by polyacrylamide gel electrophoresis. Streptomycin resistance of the parental strain and the StrR revertants was demonstrated to reside in the 30S ribosomal subunit. The second site mutations of the revertants depressed the level of streptomycin resistance in vivo and in the in vitro translation of phage SP01 messenger ribonucleic acid (mRNA) relative to the resistance exhibited by the StrR parental strain. The StrR parent grew slowly and sporulated at approximately 1% of the wild type level. The StrS revertants closely resembled the wild type strain with regard to growth and sporulation. The StrR revertants grew at rates intermediate between those of the StrR parent and wild type, and sporulated at wild type levels.