ABSTRACT
We have determined the sequences and positions of the cis elements required for proper functioning of the ARG3 promoter and proper arginine-specific control. A TATA box located 100 nucleotides upstream of the transcription start was shown to be essential for ARG3 transcription. Two sequences involved in normal arginine-mediated repression lie immediately downstream of the TATA box: an essential one (arginine box 1 [AB1]) and a secondary one (arginine box 2 [AB2]). AB1 was defined by saturation mutagenesis and is an asymmetrical sequence. A stringently required CGPu motif in AB1 is conserved in all known target sites of C6 zinc cluster DNA-binding proteins, leading us to propose that AB1 is the binding site of ARGRII, another member of the C6 family. The palindromic AB2 sequence is suggested, on the basis of published data, to be the binding site of ARGRI, possibly in heterodimerization with MCM1. AB2 and AB1 correspond respectively to the 5' and 3' halves of two adjacent similar sequences of 29 bp that appear to constitute tandem operators. Indeed, mutations increasing the similarity of the other halves with AB1 and AB2 cause hyperrepression. To mediate repression, the operator must be located close to the transcription initiation region. It remains functional if the TATA box is moved downstream of it but becomes inoperative in repression when displaced to a far-upstream position where it mediates an arginine and ARGR-dependent induction of gene expression. The ability of the ARG3 operator to act either as an operator or as an upstream activator sequence, depending on its location, and the functional organization of the anabolic and catabolic arginine genes suggest a simple model for arginine regulation in which an activator complex can turn into a repressor when able to interfere sterically with the process of transcription initiation.
