Mating-type locus control of killer toxins fromKluyveromyces lactisandPichia acaciae

Evidence for DNA cleavage caused directly by a transfer RNA-targeting toxin

The killer yeast species Pichiaacaciae produces a heteromeric killer protein, PaT, that causes DNA damage and arrests the cell cycle of sensitive Saccharomyces cerevisiae in the S phase. However, the mechanism by which DNA damage occurs remains elusive. A previous study has indicated that Orf2p, a subunit of PaT, specifically cleaves an anticodon loop of an S. cerevisiae transfer RNA (tRNA^Gln_mcm5s2UUG). This finding raised a question about whether the DNA damage is a result of the tRNA cleavage or whether Orf2p directly associates with and cleaves the genomic DNA of sensitive yeast cells. We showed that Orf2p cleaves genomic DNA in addition to cleaving tRNA in vitro. This DNA cleavage requires the same Orf2p residue as that needed for tRNA cleavage, His299. The expression of Orf2p, in which His299 was substituted to alanine, abolished the cell cycle arrest of the host cell. Moreover, the translation impairment induced by tRNA cleavage enabled Orf2p to enter the nucleus, thereby inducing histone phosphorylation.

The primary target of the killer toxin from Pichia acaciae is tRNA(Gln)

The Pichia acaciae killer toxin (PaT) arrests yeast cells in the S-phase of the cell cycle and induces DNA double-strand breaks (DSBs). Surprisingly, loss of the tRNA-methyltransferase Trm9 - along with the Elongator complex involved in synthesis of 5-methoxy-carbonyl-methyl (mcm(5)) modification in certain tRNAs - conferred resistance against PaT. Overexpression of mcm(5)-modified tRNAs identified tRNA(Gln)((UUG)) as the intracellular target. Consistently, toxin-challenged cells displayed reduced levels of tRNA(Gln) and in vitro the heterologously expressed active toxin subunit disrupts the integrity of tRNA(Gln)((UUG)). Other than Kluyveromyces lactis zymocin, an endonuclease specific for tRNA(Glu)((UUC)), affecting its target in a mcm(5)-dependent manner, PaT exerts activity also on tRNA(Gln) lacking such modification. As sensitivity is restored in trm9 elp3 double mutants, target tRNA cleavage is selectively inhibited by incomplete wobble uridine modification, as seen in trm9, but not in elp3 or trm9 elp3 cells. In addition to tRNA(Gln)((UUG)), tRNA(Gln)((CUG)) is also cleaved in vitro and overexpression of the corresponding gene increased resistance. Consistent with tRNA(Gln)((CUG)) as an additional TRM9-independent target, overexpression of PaT's tRNase subunit abolishes trm9 resistance. Most interestingly, a functional DSB repair pathway confers PaT but also zymocin resistance, suggesting DNA damage to occur generally concomitant with specific tRNA offence.

Zymocin, a composite chitinase and tRNase killer toxin from yeast

Growth inhibition of Saccharomyces cerevisiae by the plasmid-encoded trimeric (alphabetagamma) zymocin toxin from dairy yeast, Kluyveromyces lactis, depends on a multistep response pathway in budding yeast. Following early processes that mediate cell-surface contact by the chitinase alpha-subunit of zymocin, later steps enable import of the gamma-toxin tRNase subunit and cleavage of target tRNAs that carry modified U34 (wobble uridine) bases. With the emergence of zymocin-like toxins, continued zymocin research is expected to yield new insights into the evolution of yeast pathosystems and their lethal modes of action.

Homologous recombination and the yKu70/80 complex exert opposite roles in resistance against the killer toxin from Pichia acaciae

The linear plasmid (pPac1-2) encoded killer toxin (PaT) of the yeast Pichia acaciae arrests sensitive Saccharomyces cerevisiae cells in the S-phase of the cell cycle and induces mutations. Here we provide evidence for opposite effects in PaT resistance of homologous recombination (HR) and non-homologous end joining (NHEJ), the two alternative repair mechanisms acting on DNA double strand breaks (DSB). As mutants defective in genes of the RAD52 epistasis group react hypersensitive and cells lacking YKU70 or YKU80 are partially resistant, the yKu70/80 complex facilitates PaT toxicity, whereas HR is antagonistic. In contrast to yku70 and yku80, lif1 mutants, the latter being defective in the ligation step of NHEJ, are PaT sensitive, confining toxicity promoting effects of NHEJ to the DSB end binding Ku proteins. Since rad52 yku80 double mutants display strong hypersensitivity, yku80 mediated resistance depends on HR. Opposite effects of the yKu70/80 complex and HR are consistent with the occurrence of replication dependent (one sided) DSBs in PaT treated cells. Concordantly, two cellular markers signaling DSBs are induced during PaT mediated S-phase arrest, i.e. histone H2A phosphorylation and formation of subnuclear repair foci by GFP tagged recombination protein Rad52. As only moderate chromosome fragmentation could be detected by PFGE, transient occurrence and efficient in vivo repair of PaT induced DSBs is assumed. Consistent with replication dependent DSB formation induced by PaT, we demonstrate a protective function of the RecQ helicase Sgs1 and the structure specific endonuclease Mus81, both of which are considered to be involved in processing and restart of stalled replication forks.

Pichia acaciae killer system: genetic analysis of toxin immunity

The gene responsible for self-protection in the Pichia acaciae killer plasmid system was identified by heterologous expression in Saccharomyces cerevisiae. Resistance profiling and conditional toxin/immunity coexpression analysis revealed dose-independent protection by pPac1-2 ORF4 and intracellular interference with toxin function, suggesting toxin reinternalization in immune killer cells.

Yeast autonomous linear plasmid pGKL2: ORF9 is an actively transcribed essential gene with multiple transcription start points

A pair of linear plasmids, pGKL1 (8.9 kb) and pGKL2 (13.4 kb), resides in the cytoplasm of Kluyveromyces lactis killer strains. The smaller element, actually conferring the killer phenotype, strictly depends on the larger autonomous pGKL2. Here, we have examined the previously uncharacterized pGKL2 open reading frame (ORF)9 (1.34 kb). Northern analysis of a killer plasmid carrying Saccharomyces cerevisiae strain applying an ORF9-specific probe revealed a single transcript closely matching the size of the ORF9 coding region. Multiple transcriptional start points, determined by primer extension analysis, are located 16 nt downstream of a conserved sequence element regarded as the cytoplasmic promoter. In vivo disruption of pGKL2/ORF9 using the cytoplasmically expressible marker-gene LEU2* resulted in the establishment of a three-plasmid system composed of the native cytoplasmic elements pGKL1/2 and a hybrid of the latter, which only remained stable under selective conditions. The native pGKL2, however, did not segregate during prolonged subcultivations, proving an essential function of ORF9 for plasmid maintenance.