Organization and Expression of the Cell Cycle Gene, ts11, That Encodes Asparagine Synthetase

Abstract

The human ts11 gene was isolated on the basis of its ability to complement the mutation of the BHK cell cycle ts11 mutant, which is blocked in G1 at the nonpermissive temperature. This gene has now been identified as the structural gene for asparagine synthetase (AS) on the bases of sequence homology and the ability of exogenous asparagine to bypass the ts11 block. The ts11 (AS) mRNA has a size of about 2 kilobases and is induced in mid-G1 phase in human, mouse, and hamster cell lines. We have studied the organization and regulation of expression of the ts11 gene. The human ts11 gene consists of 13 exons (the first two noncoding) interspersed in a region of about 21 kilobases of DNA. Transient expression assays using the bacterial chloramphenicol acetyltransferase reporter gene identified two separate promoters: one (ts11 P1) contained in a 280-base-pair region upstream of the first exon and the other (ts11 P2) contained in the first intron, ts11 P1 produced about sixfold more chloramphenicol acetyltransferase activity than did ts11 P2 and had features of the promoters of housekeeping genes: high G+C content, multiple transcription start sites, absence of a TATA box, and presence of putative Sp1 binding sites, ts11 P2 contained a TATA sequence and other elements characteristic of a promoter, but so far we have no evidence of its physiological utilization. The ts11 gene was overexpressed in ts11 cells exposed to the nonpermissive temperature. Addition of asparagine to the culture medium led to a drastic decrease in mRNA levels and prevented G1 induction in serum-stimulated cells, which indicated that expression of the AS gene is regulated by a mechanism of end product inhibition.