Molecular characterization of transposable-element-associated mutations that lead to constitutive L-ornithine aminotransferase expression in Saccharomyces cerevisiae.

The cargB or CAR2 gene, coding for ornithine aminotransferase, was isolated by functional complementation of a cargB- mutation in Saccharomyces cerevisiae. It was used as a hybridization probe to analyse RNA and chromosomal DNA from four strains bearing cis-dominant regulatory mutations leading to constitutive, mating-type-dependent, ornithine aminotransferase synthesis. The four mutations appear to be insertions. Their size and restriction pattern suggested that they were transposable elements, Ty1. All were inserted in the same orientation with respect to the cargB gene. We cloned the cargB gene with its associated insertion from two constitutive mutants (cargB+ Oh-1 and cargB+ Oh-2). We determined the sequence of the cargB 5' region from the wild-type gene and from the two mutated genes. The DNA sequences of the extremities of the two insertions were very homologous but not identical and were similar to previously reported Ty1 element direct repeats (delta). The same five-base-pair sequence, ATATA, was found at both ends of both Ty1 elements, indicating that both Ty1 were transposed to the same site. This site is located 115 base pairs upstream from the putative cargB coding region. The 5' end of cargB transcript as determined by S1 mapping was the same in the wild-type strain and in the four mutants. The cargB transcript was not detected in the wild-type strain grown under non-induced conditions, while under the same conditions it was present in all four mutants.