Hyper-resistance to nitrogen mustard in Saccharomyces cerevisiae is caused by defective choline transport

The recessive hnm1 mutant allele is responsible for hyper-resistance to nitrogen mustard in Saccharomyces cerevisiae. Transformation with a single-copy HNM1 wild-type allele of such hyper-resistant mutants will restore wild-type sensitivity to nitrogen mustard. By contrast the presence of multi-copy vectors containing HNM1, in either a hyper-resistant hnm1 mutant or an HNM1 wild-type, will lead to a novel, mustard-sensitive phenotype unrelated to defects in DNA repair genes. Gene disruption of HNM1 revealed that this gene is non-essential for cells prototrophic for choline (CHO1) but lethal for cells with a cho1 genotype. Sensitivity to nitrogen mustard of wild-type HNM1, but not of hnm1 mutants, depends on the choline content of the growth medium, with cells grown in choline-free medium exhibiting the highest sensitivity. Sequencing of a 300 bp DNA fragment of HNM1 revealed the identity of this gene with the CTR locus, which is responsible for choline transport in Saccharomyces cerevisiae.