A met4 mutant of Saccharomyces cerevisiae was unable to transcribe a number of genes encoding enzymes of the methionine biosynthetic pathway. The sequence of the cloned MET4 gene allowed the previously sequenced flanking LEU4 and POL1 genes to be linked to MET4 into a 10 327 bp contiguous region of chromosome XIV. From the sequence and mapping of the transcriptional start points, MET4 is predicted to encode a protein of 634 amino acids (as opposed to 666 amino acids published by others) with a leucine zipper domain at the C-terminus, preceded by both acidic and basic regions. Thus, MET4 belongs to the family of basic leucine zipper trans-activator proteins. Disruption of MET4 resulted in methionine auxotrophy with no other phenotype. Transcriptional studies showed that MET4 was regulated by the general amino acid control and hence by another bZIP protein encoded by GCN4. GCN4 binding sequences are present between the divergently transcribed MET4 and LEU4 genes. Over-expression of MET4 resulted in leaky expression from the otherwise tightly regulated MET3 promoter under its control. The presence of consensus sequences for other potential regulatory elements in the MET4 promoter suggests a complex regulation of this gene.