Thymidine-requiring mutants of Salmonella typhimurium that are defective in deoxyuridine 5'-phosphate synthesis

A combination of three mutations, dcd, pyrH, and cdd, establishes thymidine (Deoxyuridine) auxotrophy in thyA+ strains of Salmonella typhimurium

The dum gene of Salmonella typhimurium was originally identified as a gene involved in dUMP synthesis (C. F. Beck et al., J. Bacteriol. 129:305-316, 1977). In the genetic background used in their selection, the joint acquisition of a dcd (dCTP deaminase) and a dum mutation established a condition of thymidine (deoxyuridine) auxotrophy. In this study, we show that dum is identical to pyrH, the gene encoding UMP kinase. The level of UMP kinase activity in the dum mutant was found to be only 30% of that observed for the dum+ strain. Thymidine prototrophy was restored to the original dum dcd mutant (KP1361) either by transduction using a pyrH+ donor or by complementation with either of two pyrH+-carrying plasmids. Thymidine auxotrophy could be reconstructed in the dum+ derivative (KP1389) by the introduction of a mutant pyrH allele. To define the minimal mutational complement necessary to produce thymidine auxotrophy in thyA+ strains, a dcd::Km null mutation was constructed. In the wild-type background, dcd::Km alone or in combination with a pyrH (dum) mutation did not result in a thymidine requirement. A third mutation, cdd (cytidine-deoxycytidine deaminase), was required together with the dcd and pyrH mutations to impart thymidine auxotrophy.

Ribonucleotides are channeled into a mixed DNA-RNA polymer by permeabilized hamster cells

Permeabilized CHEF/18 hamster cells incorporate label from [3H] cytidine diphosphate (CDP) into material that we had designated as DNA. Spyrou and Reichard reported this label was incorporated only into RNA. To resolve this discrepancy the studies reported here were performed. We demonstrate incorporation into a DNA polymer with major interspersed RNA sequences. Incorporation into deoxycytidine isolated from this product was little diluted by a great excess of unlabeled dCTP, confirming channeling under these conditions of CDP into polymerized deoxyribonucleotides.

Deoxyribonucleotide biosynthesis in yeast (Saccharomyces cerevisiae). A ribonucleotide reductase system of sufficient activity for DNA synthesis

Ribonucleotide reductase, the central enzyme of DNA precursor biosynthesis, has been isolated and characterized from baker's yeast. The enzyme activity, measured in extracts from three different, exponentially growing yeast strains, is high enough to meet the substrate requirement of DNA replication, in contrast to very low activities found in most other organisms. In thymidylate-permeable yeast cells ribonucleotide reductase activity is stimulated under both starvation and excess of intracellular dTMP. On the other hand growth of yeast in presence of 20 mM hydroxyurea did not increase enzyme activity. Yeast ribonucleotide reductase is composed of two non-identical subunits, inactive separately, of which one binds to immobilized dATP. The relative molecular mass of the holoenzyme is about 250 000. The enzyme reduces all four natural ribonucleoside diphosphates with comparable efficacy. GDP reduction requires dTTP as effector, ADP reduction is stimulated by dGTP, whereas pyrimidine nucleotide reduction is stimulated by any deoxyribonucleotide and ATP. Enzyme activity is independent of exogenous metal ions and is insensitive towards chelating agents. Hydroxyurea inactivates yeast ribonucleotide reductase in a slow reaction; half-inhibition (I50) is reached only at 2-6 mM hydroxyurea concentration. Up to 50% reactivation occurs spontaneously after removal of the inhibitor. In accord with previous attempts by others, extensive purification of the yeast enzyme has failed owing to its extreme instability in solution; the half-life of about 11 h could not be influenced by any protective measure. Taken together, yeast ribonucleotide reductase combines features known from Escherichia coli and mammalian enzymes with differing, individual properties.

A mutation in Salmonella typhimurium imparting conditional resistance to 5-fluorouracil and a bioenergetic defect: mapping of cad

The position of the genetic locus allelic with the cad-2 mutation has been located between units 14 and 15 of the linkage map of S. typhimurium. Fine structure mapping established the gene order as cad flrB nag. The genetic evidence coupled with biochemical evidence indicates that this cad locus is homologous to the ubiF gene of Escherichia coli.

Escherichia coli mutants deficient in deoxyuridine triphosphatase

Mutants deficient in deoxyuridine triphosphatase (dUTPase) were identified by enzyme assays of randomly chosen heavily mutagenized clones. Five mutants of independent origin were obtained. One mutant produced a thermolabile enzyme, and it was presumed to have a mutation in the structural gene for dUTPase, designated dut. The most deficient mutant had the following associated phenotypes: less than 1% of parental dUTPase activity, prolonged generation time, increased sensitivity to 5'-fluorodeoxyuridine, increased rate of spontaneous mutation, increased rate of recombination (hyper-Rec), an inhibition of growth in the presence of 2 mM uracil, and a decreased ability to support the growth of phage P1 (but not T4 or lambda). This mutation also appeared to be incompatible with pyrE mutations. A revertant selected by its faster growth had regained dUTPase activity and lost its hyper-Rec phenotype. Many of the properties of the dut mutants are compatible with their presumed increased incorporation of uracil into DNA and the subsequent transient breakage of the DNA by excision repair.