Relationship between yeast virus like elements and poxviruses at the transcriptional level

Abstract

Yeast cytoplasmic double-stranded DNA virus-like elements (VLEs, also known as linear plasmids) were found in a number of yeast species belonging to nine genera. The Kluyveromyces lactis pGKL1/2 plasmids, which serve as archetypes of yeast linear plasmids, are peculiar in many respects. Both plasmids are cytoplasmically localized, possess proteins covalently linked to their terminal inverted repeats and their compact genomes code for 15 genes in total including a killer toxin, two DNA polymerases, an RNA polymerase, a DNA helicase and a capping enzyme.Functions of most of the genes are putative and have not been assigned to them experimentally yet. We and others have found, that the RNA polymerase encoded by pGKL plasmids, as well as the capping enzyme and RNA helicase, are similar to the corresponding enzyme encoded by vaccinia virus (VACV). We have recently described unique structures of yeast VLEs mRNAs, we found that although these VLEs encode their own putative capping enzyme, only a few VLEs genes code for 5'-capped transcripts and that all of the VLE specific mRNAs are not 3'-polyadenylated. Surprisingly, the majority of VLE promoters give rise to uncapped mRNAs starting with short poly(A) sequences at their 5' ends that are not complementary to the plasmid DNA.