Selection and preliminary characterization of cycloleucine-resistant CHO cells affected in methionine metabolism.

Cycloleucine is in vivo a potent inhibitor of S-adenosylmethionine (SAM) biosynthesis and subsequent methylation reactions in somatic mammalian cells. Cycloleucine-resistant (CLr) clones were isolated from CHO cells by single-step selection. Their phenotype was stable when they were grown in the absence of drug. These clones appeared randomly in cultures at the frequency of 5 x 10(-6)/cell/generation, as determined by a fluctuation test. EMS mutagenesis did not significantly increase this frequency. The cycloleucine-resistant phenotype was codominant in intraspecific hybrids. Cycloleucine-resistant clones showed increased SAM pools; on the contrary, methionine pools were not significantly affected in these clones when compared to the wild-type cells. This increased SAM production was correlated with an increase of methionine adenosyltransferase (MAT) SPECIFIC ACTIVITY IN RESISTANT CLONES UNDER VARIOUS GROWm. The mechanism of posttranscriptional control of MAT biosynthesis was not affected in the cycloleucine-resistant clones nor were the kinetic properties of the enzyme modified. The genetic or epigenetic origin of this resistance mechanism is discussed.