Yeast LEU1 encodes the second enzyme in leucine biosynthesis. A 3.5-kilobase pair (kb) yeast genomic DNA fragment which complements a leu1 auxotroph was isolated by yeast transformation. After recloning into an integrating vector, a subfragment (of the 3.5-kb fragment) directs a URA3 marker to integrate at the LEU1 locus. About 1.9 kb was sequenced from the 5'-end of the 3.5-kb insert, and a long open reading frame and potential ATG start codon were located. S1 nuclease mapping showed a major start for LEU1 transcripts at 79 nucleotides upstream of the ATG codon. Northern blots with a LEU1-specific probe showed the size of the LEU1 transcript (about 2.9 kb) is consistent with the size of the enzyme and steady state levels of the transcript are sharply reduced in cells grown in the presence of an elevated leucine concentration. The latter observation correlates with the repression by leucine of LEU1 gene product levels. Other work has shown that the level of the LEU2 gene product is also repressed by leucine. Sequence comparisons between LEU1 and LEU2 show that the LEU2 5'-sequences which are cognate to leucine are not found in LEU1; and three blocks of nucleotide sequence homology between LEU1 and LEU2 occur in the 330 nucleotides upstream of the respective start codons.