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Previous studies showed that the enzyme (phosphoribosyltransferase) which catalyzes the first step of the histidine pathway in Salmonella typhimurium plays a role in regulation of the histidine operon. Since histidyl transfer ribonucleic acid (His-tRNA) is required for repression of the histidine operon, we considered the possibility that the role of phosphoribosyltransferase might be realized through an interaction with His-tRNA. One prediction inherent in this idea is that the enzyme should interact with His-tRNA in vitro. Evidence is presented for such an interaction. Binding of 3 H-His-tRNA to purified phosphoribosyltransferase was tested on Sephadex columns and on nitrocellulose filters. The enzyme was found to have a high affinity for tRNA. Comparing the binding of 3 H-His-tRNA with that of tRNA aminoacylated with other 3 H-amino acids disclosed that the binding of the histidyl species of tRNA is favored over that of other species and is dependent upon magnesium-ion concentration.

The factor which corrects the abnormal mucopolysaccharide metabolism of Hurler fibroblasts (Hurler factor) has been purified 1000-fold from normal human urine. The Hurler factor behaves at high salt concentration as a protein of molecular weight of 87,000, as determined by chromatography on Sephadex G-200; at low salt concentration, it tends to aggregate and become inactive. Two major peaks of corrective activity can be separated by hydroxylapatite chromatography. Experiments with whole cells indicate that Hurler factor accelerates degradation of stored sulfated mucopolysaccharide and that the effect of exogenously supplied Hurler factor persists in Hurler fibroblasts with a half-life of at least 9 days. Hurler factor is distinct from the common lysosomal hydrolases, including β-galactosidase.

Previous studies suggested that phosphoribosyltransferase, which catalyzes the first step of the pathway for histidine biosynthesis in Salmonella typhimurium and which is sensitive to inhibition by histidine, plays a role in repression of the histidine operon. Recently, we showed that the enzyme has a high affinity for histidyl transfer ribonucleic acid (His-tRNA), which is known to participate in the repression process. In the present study, we have investigated further the interaction between the enzyme and His-tRNA. We found that His-tRNA binds at a site on phosphoribosyltransferase distinct from the catalytic site and the histidine-sensitive site; that the substrates of the enzyme inhibit the binding of His-tRNA, whereas histidine does not do so; that, once a complex has been formed between phosphoribosyltransferase and His-tRNA, the substrates of the enzyme decrease the stability of the complex, whereas histidine is without effect; and that purified phosphoribosyltransferase which has a defect in its inhibition by histidine (produced by mutation) displays an altered ability to bind His-tRNA, a finding which may be a reflection of the fact that mutants producing such a defective enzyme display an alteration of the repression process.

This paper will summarize recent studies on the nature and function of those proteins which are deficient in the mucopolysaccharidoses.

The genetic mucopolysaccharidoses are a rare but potentially most instructive group of diseases. For at the root of the complex anatomical, neurological, and biochemical aberrations seen in each of the mucopolysaccharidoses lies a single malfunctioning protein—the product of a single genetic error. To the investigator, the problem is therefore crystallized: identify the defective protein and discover its normal function. Only then will it be possible to understand the exact pathophysiology of these diseases.