Selectable mutations altering two mechanisms of mammalian K+ transport are dominant

Somatic cell mutants with altered K+ transport have previously been isolated from mutagenzied LMTK- cells for their ability to grow at subthreshold low-potassium concentrations (0.2 mM). These mutants fall into two classes: one class, LTK-5, possesses a functionally altered furosemide-sensitive Na+-K+-Cl- cotransport system and the other, LTK-1, an altered K+-conducting channel. Somatic cell hybrids have been formed between each of these cell lines and a wild-type L-cell line, making use of complementing selectable marker mutations carried by these parents, to establish the dominance of the K+ transport mutations. Hybrids were isolated and studied in two ways: clonal hybrid cell lines were selected in a manner unbiased toward their K+ transport phenotype, which was later assayed; and the number of independent hybrids arising in this single-selective condition was compared with the number arising in a condition which is double selective for the mutant phenotype as well. By both assays, hybrids formed with LTK-1 or LTK-5 as a parent uniformly exhibited the mutant phenotype by growth and cloning, whereas control hybrids with LMTK- as parent never did. This demonstrates both transport mutations to be dominant and thus potentially isolatable.