Polyribosomes of the histidine operon in Salmonella typhimurium

An enzyme of histidine biosynthesis, phosphohistidinol phosphatase, was located in polyribosomes of Salmonella typhimurium after derepression of the histidine operon. A technique of gel filtration on Sepharose was used to remove the free phosphohistidinol phosphatase from bacterial lysates which interfered with the detection of polyribosome-bound enzyme. The sedimentation coefficient of polyribosomes containing phosphohistidinol phosphatase is of the order of 400 to 550 s which suggests that they are polycistronic. This supposition is supported by the observation that in a mutant carrying a deletion of part of the histidine operon, phosphohistidinol phosphatase is associated with polyribosomes of markedly lower sedimentation coefficient. The level of polyribosome-bound phosphohistidinol phosphatase is regulated by the concentration of L-histidine in the growth medium in a strain containing a functional operator gene, and is insensitive to histidine in an operator constitutive mutant, which suggests that the expression of the histidine operon is at least partly regulated at the level of transcription.