Relaxed mutants of Serratia marcescens SM-6. Biochemical traits and relevance of the rel+ allele for the formation of exoenzymes.

Serratia marcescens SM-6 when starved for a required amino acid stops synthesizing protein and RNA and accumulates two nucleotides which co-chromatograph with ppGpp and pppGpp. These features are characteristic of bacterial strains with stringent RNA control (rel+). Two independent mutants were isolated which resemble relaxed (relA) mutants of Escherichia coli; they continue to synthesize RNA and accumulate neither ppGpp nor pppGpp when deprived of the required amino acid. The extracellular enzyme activities (nuclease, protease, lipase) of the relaxed mutants are about the same as those of the parental stringent strain when studied under standard growth conditions. Exoenzyme-deficient (nuc;prt) and exoenzyme-hyperproducing (nucsu) mutants were isolated from both stringent and relaxed strains of S. marcencens SM-6 and no change of the cellular ability to form ppGpp and pppGpp could be observed. From these results it appears that the formation of exoenzymes of S. marcescens SM-6 is independent of stringent/relaxed RNA control.