CIS-ACTING AND TRANS-ACTING ELEMENTS INVOLVED IN AMINO-ACID REGULATION OF ASPARAGINE SYNTHETASE GENE-EXPRESSION

We have previously shown that asparagine synthetase (AS) mRNA expression can be dramatically up-regulated by asparagine deprivation in ts11 cells, mutants of BHK hamster cells which encode a temperature-sensitive AS. The expression of AS mRNA was also induced upon starvation for one of several essential amino acids in HeLa cells. We also showed that regulation of AS mRNA expression by amino acid concentration has both transcriptional and posttranscriptional components. Here we report the analysis of the elements in the human AS promoter region important for its basal activity and activation by amino acid starvation. Our results indicate that a DNA fragment spanning from nucleotides -164 to +44 of the AS promoter is sufficient for uninduced and induced gene expression. Mutations in a region located 15 to 30 bp downstream from the major transcription start site that shows good homology to a sequence in the first exon of c-fos implicated as a negative regulatory element resulted in a significant increase in basal gene expression but did not affect regulation. Interestingly, this region binds single-stranded-DNA-binding proteins that are specific for the AS coding strand. Mutations in either one of two putative binding sites for transcription factor Spl, in a region of approximately 60 bp where many minor RNA start sites are located, or at the major transcription start site decreased promoter activity, but significant induction by amino acid starvation was still observed. Strikingly, mutations centered around nucleotide -68 not only decreased the basal promoter activity but also abolished amino acid regulation. This DNA region contains the sequence 5'-CATGATG-3', which we call the amino acid response element (AARE), that can bind a factor(s) present in HeLa cells nuclear extracts that is not capable of binding to an AS promoter with mutations or deletions of the AARE. This finding is in line with the hypothesis that transcriptional activation of AS gene expression is mediated through the binding of a positive regulatory element. We did not detect changes in the level of binding of this factor to the AARE by using nuclear extracts from HeLa cells grown under starved conditions, suggesting that activation of this factor(s) results from posttranslational modification or complexing with other proteins that do not affect its DNA-binding properties.