Killer toxin production in Pichia acaciae is associated with linear DNA plasmids.

We have identified a strain of the yeast Pichia acaciae which produces a "killer" toxin active against the yeast Debaryomyces tamarii. The killer phenotype was associated with the presence of two DNA plasmids, pPacl-1 (13.6 kilobase pairs) and pPacl-2 (7.3 kilobase pairs). P. acaciae strains, cured of these plasmids by irradiation with ultraviolet light, lacked killer activity and were sensitive to toxin produced by the parental strain. A partially cured strain, GS-1215, missing only the smaller plasmid, pPacl-2, also exhibited loss of both toxin activity and immunity. Exonuclease studies revealed that both plasmids were linear double-stranded DNA molecules with 5' protected ends. The P. acaciae system differs from that of the well-studied Kluyveromyces lactis "killer" system both in the range of susceptible strains and in the sizes of the plasmids involved. Our studies contradict previous reports that Pichia killer systems are invariably chromosomal.