No abstract

No abstract

No abstract

The dihydrofolate reductase gene of Saccharomyces cerevisiae has been isolated by selection of trimethoprim resistant Escherichia coli transformed with a gene bank of yeast DNA in plasmid pBR322. From 9.2 kilobase pair BamHI DNA fragment this gene has been localized to a 1.76 kb fragment, the restriction map of which appears different from those reported for the E. coli and the mouse dihydrofolate reductase genes.The enzyme encoded by the chimeric plasmid was established as yeast dihydrofolate reductase by its sensitivity to antifolates in vivo through growth studies and in vitro by enzyme assay. Since, the expression of this gene occurs independent of its orientation within the chimeric plasmid, the 1.76 kb fragment may contain functional regulatory sequences in addition to the structural sequences for yeast dihydrofolate reductase.

A kinetic analysis of L-cystine uptake in wild-type Salmonella typhimurium indicates the presence of at least two, and possibly three, separate transport systems. CTS-1 accounts for the majority of uptake at 20 ,uM L-cystine, with a Vmax of 9.5 nmol/min per mg and a Km of 2.0 ,M; CTS-2 is a low-capacity, higher-affinity system with a Vm. of 0.22 nmol/min per mg and aKi of 0.05 pM;