Messenger RNAs transcribed from yeast linear cytoplasmic plasmids possess unconventional 5’ and 3’ UTRs and suggest a novel mechanism of translation

Linear plasmids with almost identical compact genetic organization have been found in the cytoplasm of yeast species from nine genera. We employed pGKL1,2 plasmids fromKluyveromyces lactisas a model to investigate the previously unstudied transcriptome of yeast cytoplasmic linear plasmids. We performed 5’ and 3’ RACE analysis of all the pGKL1,2 mRNAs and found them not 3’ polyadenylated and containing mostly uncapped 5’ poly(A) leaders that are not complementary to the plasmid DNA. The degree of 5’ capping and/or 5’ polyadenylation is specific to each gene and is controlled by the corresponding promoter regions. We refined the description of the pGKL1,2 promoters and found new alternative promoters of several genes. We also provide evidence thatK2ORF3encodes an mRNA cap guanine-N7-methyltransferase and that 5’ capped pGKL1,2 transcripts contain N7-methylated caps. Translation of pGKL1,2 transcripts is enhanced inIsm1Δandpab1Δstrains and is independent of eIF4E and Pab1 translation factors. We suggested a model of a primitive regulation of pGKL1,2 plasmids gene expression where degree of 5’ mRNA capping, degree of 5’ non-template polyadenylation and presence of negative regulators as PAB1 and Lsm1 play an important role. Our data also suggest a close relationship between linear plasmids and poxviruses.