A transcriptional barrier to expression of cloned toxin genes of the linear plasmid k1 ofKluyveromyces lactis: evidence that native k1 has novel promoters

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Abstract
The killer toxin of Kluyvercmyces lactis consists of three polypeptides encoded by the linear plasmid k1. We re-introduced the entire k1 sequence cloned on a circular replicating plasmid, into K. lactis strains lacking k1, and found that the resulting transfonnants did not produce toxin. The barrier to eacpresaion was found to be transcriptional: the four transcripts of native k1 were absent, and instead storter, aberrant K1 transcripts were made. determined the precise initiation sites of the four transcripts of native k1: these had wry sbort untranslated leaders and mapped a 14 Eream of an ”upstream czwiserved sa“ (UCS). It appears that k1 has novel prcaxters which are inactive on circular plaenids which replicate in the riicleus. This is consistent with the suggestion that native k1 resides in the cytoplasm.