Expression of the yeast LEU4 gene is subject to four different modes of control.

A translational fusion of yeast LEU4 and Escherichia coli lacZ which contains 679 bp of the LEU4 5'-flanking region and the first two codons of LEU4 was used to study LEU4 expression. Eight recipient strains with different genetic backgrounds, transformed with a plasmid containing the fusion, were grown under a variety of conditions, and beta-galactosidase activity was measured. Evidence was obtained for at least four modes of expression of LEU4: general amino acid control, leucine-specific control, basal level expression, and branched-chain amino acid-mediated repression. Determination of steady-state levels of LEU4 mRNA suggested that LEU4 expression is regulated transcriptionally.