Expression of asparagine independence in variants of ICR 2A haploid frog cells.
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.
Hepfer
C E
CE
Department of Biology, Millersville University, Pennsylvania 17551.
Little
W A
WA
Hengst
J A
JA
Freed
J J
JJ
D000818:Animals; D001216:Asparagine / Q000378:metabolism*; D001217:Aspartate-Ammonia Ligase / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D004247:DNA / Q000032:analysis / Q000378:metabolism; D014644:Genetic Variation; D006238:Haploidy; D007621:Karyotyping; D008745:Methylation; D009153:Mutagens / Q000494:pharmacology; D012333:RNA, Messenger / Q000032:analysis; D011894:Rana pipiens / Q000196:embryology
D009153:Mutagens; D012333:RNA, Messenger; D001216:Asparagine; D004247:DNA; D001217:Aspartate-Ammonia Ligase