Genome organization of the linear cytoplasmic element pPE1B from Pichia etchellsii

The linear cytoplasmic element pPE1B from Pichia etchellsii CBS2011 (synonym Debaryomyces etchellsii) was totally sequenced. It consists of 12835 bp and has a remarkable high A+T content of 77.3%. The termini of pPE1B were found to consist of inversely orientated identical nucleotide repetitions 161 base pairs long, to which proteins are probably covalently linked at the 5' ends. Ten putative genes (open reading frames, ORFs) were identified, covering 96.5% of the total sequence. The predicted polypeptides correspond to proteins encoded by ORFs 2-11 of the linear plasmids pGKL2 of Kluyveromyces lactis and pSKL of Saccharomyces kluyveri. ORF1, existing on both latter elements, is lacking on pPE1B. An upstream conserved sequence motif (UCS) is located at the expected distance from the start codon of each of the 10 ORFs. As the arbitrarily chosen UCS6 was able to drive expression of a reporter gene in the heterologous pGKL-encoded killer system of K. lactis, extranuclear promoter function is probable. The almost congruent genome organization of pPE1B and other autonomous linear yeast plasmids sequenced so far, i.e. pGKL2 and pSKL, suggests a common, presumably viral, ancestor.