Hepfer C E, Little W A, Hengst J A, Freed J J
Department of Biology, Millersville University, Pennsylvania 17551.
J Cell Physiol. 1989 Apr;139(1):175-80. doi: 10.1002/jcp.1041390124.
Properties of the change from asparagine dependence (asn-) to independence (asn+) were investigated in the androgenetic haploid frog cell line ICR 2A. Two types of asn+ variants arose spontaneously during culture. Glutamine-dependent asparagine synthetase (AS) activity, found to be deficient in asn- cells, was repressed by asparagine in one type of variant and expressed constitutively in the other. No quantitative differences in AS-specific DNA sequences or changes in ploidy were evident between asn+ and asn- cells. The asn+ frequency in ICR 2A populations, not dramatically influenced by chemical mutagens, was increased 130-fold by exposure to 5-azacytidine. The methylation of CCGG sequences at the 5' end of the AS structural gene was found to be reduced equally in both types of asn+ variant. These results indicate that decreased DNA methylation is essential but not necessarily sufficient for the expression of AS activity in this frog cell system.
在雄核发育单倍体青蛙细胞系ICR 2A中研究了从天冬酰胺依赖型（asn-）转变为非依赖型（asn+）的特性。在培养过程中自发出现了两种类型的asn+变体。在一种变体中，天冬酰胺依赖型天冬酰胺合成酶（AS）活性在asn-细胞中被发现缺乏，它被天冬酰胺抑制，而在另一种变体中则组成性表达。在asn+和asn-细胞之间，AS特异性DNA序列没有明显的数量差异，倍性也没有变化。ICR 2A群体中的asn+频率不受化学诱变剂的显著影响，但通过暴露于5-氮杂胞苷可增加130倍。在两种类型的asn+变体中，AS结构基因5'端CCGG序列的甲基化都同样减少。这些结果表明，DNA甲基化降低对于该青蛙细胞系统中AS活性的表达是必不可少的，但不一定是充分的。
