Promoter activity associated with the left inverted terminal repeat of the killer plasmid k1 from yeast

The killer plasmid k1 of Kluyveromyces lactis has terminal inverted repeats of 202 base pairs (bp). The left terminal repeat is contiguous to the transcribed open reading frame, ORF1, which is supposed to code for a DNA polymerase. A 266-bp fragment (called Pk1) containing most of the terminal repeat sequence was isolated and examined for promoter activity. Pk1 was fused, in either original or inversed orientation, with a promoter-less lacZ gene of E coli and a promoter-less G418 resistance gene of Tn903. These fusions were introduced into a pKD1-derived circular vector, and transformed into a lactose-negative (lac4), and a G418-sensitive K lactis host. Lac+ and G418-resistant transformants were obtained with either orientation of Pk1. The promoter activity of Pk1 fragment was independent of the presence or absence of killer plasmids. It is not known whether Pk1 can also function bidirectionally on the natural k1 plasmid. The possible functions of Pk1 for killer plasmid gene expression and plasmid replication are discussed.