Two related regulatory sequences are required for maximal induction of Saccharomyces cerevisiae his3 transcription.

In Saccharomyces cerevisiae, the coordinate induction of his3 and other amino acid biosynthesis genes is mediated by the binding of GCN4 activator protein to specific promoter sequences. The his3 regulatory region contains the sequence TGACTC, which with some variation is repeated six times upstream of the mRNA initiation site. The requirements for maximal his3 induction were examined with a series of sequential 5' deletion mutations as well as a set of small internal deletions. Deletions encroaching as far downstream as position -142 behave indistinguishably from the wild-type gene, thus indicating that the two proximal copies of the regulatory sequence are sufficient for maximal induction. Deletions with breakpoints between -137 and -99 confer inducibility, but not to the normal wild-type level. A deletion ending immediately upstream of the proximal TGACTC sequence (position -99) shows some constitutive expression that is independent of the gcn4 gene product. Deletions extending to -94 or beyond do not produce detectable levels of his3 mRNA. Small internal deletions that only remove the proximal regulatory sequence and a 1-base-pair deletion of the thymine residue at -99 abolish induction, but do not affect the basal level of transcription. These results indicate that the proximal copy between -99 and -94 is absolutely required for his3 induction, whereas the copy between -142 and -137 is required only for the maximal level of induction and is inactive by itself. From these and other observations, we suggest the possibility that these related regulatory sequences may be targets for two distinct proteins.