Regulation of de novo purine biosynthesis in normal and 8-azaguanine-resistant Chinese hamster cells

Characterization of HAT- and HAsT-resistant HPRT mutant clones of V79 Chinese hamster cells

HPRT mutant clones of V79 Chinese hamster cells, isolated after 6-thioguanine (6TG) selection, normally exhibit sensitivity to growth in medium containing the folic acid inhibitor aminopterin or the glutamine analogue L-azaserine (e.g., HAT or HAsT medium). However, it has been shown that some HPRT- clones are resistant to both HAT and HAsT medium. The present study was undertaken to investigate whether any common structural gene alteration exists for such 6TGr-HATr-HAsTr clones. Four clones were studied, 1 of spontaneous origin, 2 induced by a low dose of MNU and 1 EMS-induced. In contrast to wild-type cells and a mutant clone carrying a complete deletion of the HPRT gene, these 4 investigated 6TGr-HATr-HAsTr clones all showed an enhanced incorporation of exogenous 3H-hypoxanthine in the presence of aminopterin and L-azaserine suggesting that these clones carry mutations in the structural part of the HPRT gene. Sequence analysis of PCR-amplified HPRT cDNA from these mutants showed that the spontaneous and the 2 MNU-induced mutant clones lacked exon 4, while the EMS-induced mutant had a GC to AT transition in exon 6. Southern blot analysis of genomic DNA after digestion with BglII, EcoRI and PstI showed no changes in fragment patterns as compared to the wild type. Further sequence analysis of PCR-amplified genomic DNA using exon 4-specific primers showed that all these 3 mutants had an AT to GC or GC to AT transition in exon 4, but had no alterations in the splice sites of exon 4. Based on their characteristics of hypoxanthine incorporation, the present mutant clones fit the model for the proposed functional domains of the HPRT protein.

Purine biosynthesis and its regulation in Neurospora crassa

Purine biosynthesis and its regulation was studied in Neurospora crassa by the incorporation of label from [14C]formate into total cellular purines. In general, the purine biosynthesis resulted in slightly more cellular guanine than adenine nucleotides. The acid-soluble pool however, contained more adenine compounds than guanine. Exogenous adenine was found to be an effective regulatory of the proximal steps of the de novo biosynthesis, while both adenine and guanine were equally effective in regulating the branch point activities. 6-Methyl purine inhibited the proximal steps of the purine synthesis more effectively than the branch point leading to adenine biosynthesis. A 6-methyl purine resistant mutant, Mepr-10, with defective adenine phosphoribosyl pyrophosphate transferase showed no inhibition of purine synthesis by 6-methyl purine, while 6-methyl purine resistant strains Mepr-3 and Mepr-1 showed partial inhibition. It has been suggested that Mepr-3 and Mepr-1 may be mutants of glutamine amidotransferase with altered affinities for 6-methyl purine. The rate of purine biosynthesis increased during the first 8 h of incubation of conidia in minimal medium, after which it declined even though the growth continued.