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Isolation of somatic cell mutants with specified alterations in hypoxanthine phosphoribosyltransferase
Abstract:
Cellular resistance to 6-thioguanine is almost always associated with a complete loss of hypoxanthine phosphoribosyltransferase (HPRT) activity, while resistance to 8-azaguanine has frequently been shown to occur independently of any changes in the HPRT activity. As a result, mutant cells selected for resistance to 6-thioguanine are also resistant to 8-azaguanine, but cells selected for 8-azaguanine resistance are not necessarily cross-resistant to 6-thioguanine. Our previous studies demonstrated that this difference is due to differential utilization of the two purine analogs by HPRT in the cells. In this paper we describe a novel selective procedure for the systematic isolation of cellular mutants that contain wild-type levels of HPRT, are sensitive to 6-thioguanine, and yet are able to survive in medium containing both HAT and a high level of 8-azaguanine. We also present evidence which shows that such mutants arise through a mutation that specifically alters the HPRT molecule so that the enzyme no longer recognizes 8-azaguanine as a substrate, while remaining catalytically functional with hypoxanthine and 6-thioguanine. Mutants of this type may be useful as a marker in gene-transfer experiments.
Insights
Researchers identified new cellular mutants resistant to 8-azaguanine but sensitive to 6-thioguanine, despite having normal hypoxanthine phosphoribosyltransferase (HPRT) activity. These mutants offer potential as markers in gene-transfer studies.
Area of Science:
- Biochemistry
- Molecular Biology
- Genetics
Background:
- Cellular resistance to purine analogs like 6-thioguanine (6-TG) and 8-azaguanine (8-AG) is often linked to hypoxanthine phosphoribosyltransferase (HPRT) activity.
- Loss of HPRT activity typically confers resistance to 6-TG, leading to cross-resistance to 8-AG.
- However, 8-AG resistance can occur independently of HPRT activity, indicating differential substrate utilization by the enzyme.
Purpose of the Study:
- To develop a novel method for isolating cellular mutants with wild-type HPRT levels that are sensitive to 6-TG but resistant to 8-AG.
- To investigate the molecular basis of this specific resistance phenotype.
- To explore the potential utility of these mutants as markers in gene-transfer experiments.
Main Methods:
- Utilized a selective procedure to isolate mutants resistant to 8-azaguanine in the presence of HAT medium, while maintaining sensitivity to 6-thioguanine.
- Characterized the HPRT activity and substrate specificity in the isolated mutants.
- Analyzed the mutation's effect on the HPRT enzyme's interaction with purine analogs.
Main Results:
- Successfully isolated cellular mutants exhibiting wild-type HPRT levels but showing specific resistance to 8-azaguanine and sensitivity to 6-thioguanine.
- Demonstrated that these mutants possess an altered HPRT molecule that no longer recognizes 8-azaguanine as a substrate.
- Confirmed that the mutated HPRT enzyme remains catalytically functional for hypoxanthine and 6-thioguanine.
- Provided evidence for a specific mutation affecting HPRT's substrate recognition.
Conclusions:
- A novel class of cellular mutants with altered HPRT substrate specificity has been identified.
- These mutants provide a tool to dissect the roles of HPRT in purine analog metabolism.
- The identified mutants may serve as valuable markers in gene-transfer and other molecular biology applications.