A BASIC HELIX-LOOP-HELIX-LEUCINE ZIPPER TRANSCRIPTION COMPLEX IN YEAST FUNCTIONS IN A SIGNALING PATHWAY FROM MITOCHONDRIA TO THE NUCLEUS

The expression of some nuclear genes in Saccharomyces cerevisiae, such as the CIT2 gene, which encodes a glyoxylate cycle isoform of citrate synthase, is responsive to the functional state of mitochondria, Prev ious studies identified a basic helix-loop-helix-leucine zipper (bHLH/ Zip) transcription factor encoded by the RTG1 gene that is required fo r both basal expression of the CIT2 gene and its increased expression in respiratory deficient cells, Here, we describe the cloning and char acterization of RTG3, a gene encoding a 54-kDa bHLH/Zip protein that i s also required for CIT2 expression, Rtg3p binds together with Rtg1p t o two identical sites oriented as inverted repeats 28 bp apart in a re gulatory upstream activation sequence element (UAS(r)) in the CIT2 pro moter, The core binding site for the Rtg1p-Rtg3p heterodimer is 5'-GGT CAC-3', which differs from the canonical E-box site, CANNTG, to which most other bHLH proteins bind, We demonstrate that both of the Rtg1p-R tg3p binding sites in the UAS, element are required in vivo and act sy nergistically for CIT2 expression, The basic region of Rtg3p conforms well to the basic region of most bHLH proteins, whereas the basic regi on of Rtg1p does not, These findings suggest that the Rtg1p-Rtg3p comp lex interacts in a novel way with its DNA target sites.