2 SACCHAROMYCES-CEREVISIAE GENES WHICH CONTROL SENSITIVITY TO G(1) ARREST INDUCED BY KLUYVEROMYCES-LACTIS TOXIN

The Kluyveromyces lactis toxin causes an arrest of sensitive yeast cel ls in the G(1) phase of the cell division cycle. Two complementary gen etic approaches have been undertaken in the yeast Saccharomyces cerevi siae to understand the mode of action of this toxin. First, two sequen ces conferring toxin resistance specifically in high copy number have been isolated and shown to encode a tRNA(3)(Glu) and a novel polypepti de. Disruption of the latter sequence in the yeast genome conferred to xin resistance and revealed that it was nonessential, chile the effect of the tRNA(3)(Glu) was highly specific and mediated resistance by af fecting the toxin's target. An alpha-specific, copy number-independent suppressor of toxin sensitivity was also isolated and identified as M ATa, consistent with the observation that diploid cells are partially resistant to the toxin. Second, in a comprehensive screen for toxin-re sistant mutants, representatives of 13 complementation groups have bee n obtained and characterized to determine whether they are altered in the toxin's intracellular target. Of 10 genes found to affect the targ et process, one (KT112) was found to encode the novel polypeptide prev iously identified as a multicopy resistance determinant. Thus, both lo ss of KT112 function and elevated KT112 copy number can cause resistan ce to the and K. lactis toxin.