Physical and biochemical characterization of the cloned LYS5 gene required for alpha-aminoadipate reductase activity in the lysine biosynthetic pathway of Saccharomyces cerevisiae

Yeast telomeres exert a position effect on recombination between internal tracts of yeast telomeric DNA

In Saccharomyces cerevisiae, proximity to a telomere affects both transcription and replication of adjacent DNA. In this study, we show that telomeres also impose a position effect on mitotic recombination. The rate of recombination between directly repeated tracts of telomeric C1-3A/TG1-3 DNA was reduced severely by proximity to a telomere. In contrast, recombination of two control substrates was not affected by telomere proximity. Thus, unlike position effects on transcription or replication, inhibition of recombination was sequence specific. Moreover, the repression of recombination was not under the same control as transcriptional repression (telomere position effect; TPE), as mutations in genes essential for TPE did not alleviate telomeric repression of recombination. The reduction in recombination between C1-3A/TG1-3 tracts near the telomere was caused by an absence of Rad52p-dependent events as well as a reduction in Rad1p-dependent events. The sequence-specific repression of recombination near the telomere was eliminated in cells that overexpressed the telomere-binding protein Rap1p, a condition that also increased recombination between C1-3A/TG1-3 tracts at internal positions on the chromosome. We propose that the specific inhibition between C1-3A/TG1-3 tracts near the telomere occurs through the action of a telomere-specific end-binding protein that binds to the single-strand TG1-3 tail generated during the processing of recombination intermediates. The recombination inhibitor protein may also block recombination between endogenous telomeres.

The LYS5 gene of Saccharomyces cerevisiae

The LYS2 and LYS5 genes of Saccharomyces cerevisiae together encode the 180-kDa alpha-aminoadipate reductase (AAR) in the biosynthetic pathway of lysine. The 4.8-kb LYS2 gene encodes the 155-kDa subunit of AAR. The complete nucleotide (nt) sequence of the 1.1-kb LYS5 gene is presented in this report. It contains a single continuous open reading frame of 816 nt encoding a 272-amino-acid, 30.6-kDa polypeptide.

Molecular properties of the lys1+ gene and the regulation of alpha-aminoadipate reductase in Schizosaccharomyces pombe

The alpha-aminoadipate pathway for the biosynthesis of lysine is unique to fungi. Molecular properties of the cloned lys1+ gene and the regulation of the encoded alpha-aminoadipate reductase (AAR) were investigated in the fission yeast Schizosaccharomyces pombe. A 5.2-kb HindIII-EcoRI fragment of S. pombe DNA, containing a functional lys1+ gene and a promoter, was subcloned to make the 10.7-kb plasmid pLYS1H. A nested 1.778-kb HindIII-EcoRI DNA fragment that complemented the lys1-131 mutant phenotype was sequenced from the plasmid pLYS1D, and shown to contain an open reading frame (ORF) of 470 amino acids, preceded by putative POLII promoter elements (TATA and CCAAT box elements, and two potential yeast GCN4-binding motifs) within 368 bp upstream of the start codon. This ORF shared with the corresponding region of the isofunctional AAR of Saccharomyces cerevisiae 49% amino-acid identity (62% similarity) overall, within which were smaller regions of marked sequence conservation. One such region coincided (95% identity) with a putative AMP-binding domain motif identified in the AAR of S. cerevisiae. In wild-type S. pombe, AAR activity from cells grown in lysine-supplemented minimal or YEPD media was less than the activity of cells grown in minimal medium. The AAR of S. pombe was more sensitive to feedback inhibition by lysine in vitro than the AAR of S. cerevisiae. These results show the effects of extensive evolutionary divergence on the structure and expression of a pivotal enzyme in the alpha-aminoadipate pathway. Presumably, delineated regions of strong sequence conservation correspond to discrete domains essential to AAR function.

Physical and functional characterization of the cloned lys1+ gene of Schizosaccharomyces pombe

The alpha-aminoadipate pathway for the biosynthesis of lysine is present in yeast and other higher fungi. The lys2 and lys5 mutants of Saccharomyces cerevisiae as well as the lys1- and lys7-mutants of Schizosacharomyces pombe are blocked at the alpha-aminoadipate reductase step of this pathway. The cloned lys1+ gene in the plasmid pLYS1 isolated from a S. pombe genomic library complemented lys1-mutant of S. pombe. The cloned LYS2 gene in the plasmid YEp620 and the LYS5 gene in the plasmid pSC5 of S. cerevisiae exhibited heterologous complementation of lys1- and lys7-mutants, respectively, of S. pombe. The homologous lys1+ transformed cells exhibited five fold higher alpha-aminoadipate reductase activity while the heterologous lys1+ and lys7+ transformed cells exhibited much less activity than the wild type cells. The DNA insert of the plasmid pLYS1 was determined to be 16.7 kb long and the lys1+ gene has been subcloned within a 9.1 kb Clal-Clal DNA insert of the recombinant plasmids pLYS1B and pLYS1C. The restriction pattern for 12 enzymes of the 9.1 kb DNA insert, (Apal, Aval, BamHI, Clal, EcoRI, EcoRV, HindIII, Hpal, Pstl, Pvull, Sphl, and Xbal), exhibited no obvious similarity to that of the LYS2 gene of S. cerevisiae. A 1.7 kb EcoRI-HindIII DNA fragment of pLYS1B and pLYS1C complemented the lys1-131 mutation in an integrative transformation. Although the lys1+ gene of S. pombe is isofunctional to the LYS2 gene of S. cerevisiae, the restriction sites, and expression of these two genes exhibited considerable divergence.