Summary: An strain which is temperature-sensitive for growth due to a mutation(-2)causing a defective phosphoribosyl diphosphate(PRPP)synthase has been characterized. The temperature-sensitive mutation was mapped to a 276 bp dIII-HII DNA fragment located within the open reading frame specifying the PRPP synthase polypeptide. Cloning and sequencing of the mutant allele revealed two mutations. One, a G→A transition, located in the ninth codon, was responsible for the temperature-conditional phenotype and resulted in a serine residue at this position. The wild-type codon at this position specified a glycine residue that is conserved among PRPP synthases across a broad phylogenetic range. Cells harbouring the glycine-to-serine alteration specified by a plasmid contained approximately 50% of the PRPP synthase activity of cells harbouring a plasmid-borne wildtype allele, both grown at 25°C. The mutant enzyme had nearly normal heat stability, as long as it was synthesized at 25°C. In contrast, there was hardly any PRPP synthase activity or anti-PRPP synthase antibody cross-reactive material present in cells harbouring the glycine to serine alteration following temperature shift to 42°C. The other mutation was aC→T transition located 39 bp upstream of the G→A mutation, i.e. outside the coding sequence and close to the Shine-Dalgarno sequence. Cells harbouring only the C→T mutation in a plasmid contained approximately three times as much PRPP synthase activity as a strain harbouring a plasmid-borne wild-type allele. In cells harbouring both mutations, the C→T mutation appeared to compensate for the G→A mutation by increasing the amount of a partially defective enzyme at the permissive temperature.


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