The Long Terminal Repeat Retrotransposons Tf1 and Tf2 of Schizosaccharomyces pombe

Abstract

The long terminal repeat (LTR) retrotransposons Tf1 and Tf2 of Schizosaccharomyces pombe are active mobile elements of the Ty3/gypsy family. The mobilization of these retrotransposons depends on particle formation, reverse transcription and integration, processes typical of other LTR retrotransposons. However, Tf1 and Tf2 are distinct from other LTR elements in that they assemble virus-like particles from a single primary translation product, initiate reverse transcription with an unusual self-priming mechanism, and, in the case of Tf1, integrate with a pattern that favors specific promoters of RNA pol II-transcribed genes. To avoid the chromosome instability and genome damage that results from increased copy number, S. pombe applies a variety of defense mechanisms that restrict Tf1 and Tf2 activity. The mRNA of the Tf elements is eliminated by an exosome-based pathway when cells are in favorable conditions whereas nutrient deprivation triggers an RNA interference-dependent pathway that results in the heterochromatization of the elements. Interestingly, Tf1 integrates into the promoters of stressinduced genes and these insertions are capable of increasing the expression of adjacent genes. These properties of Tf1 transposition raise the possibility that Tf1 benefits cells with specific insertions by providing resistance to environmental stress.

Diverse transposable element landscapes in pathogenic and nonpathogenic yeast models: the value of a comparative perspective

“…LTR retrotransposons comprise 1.1% of the reference genome [23,83]. There are 249 LTRs or LTR fragments, 13 full-length retrotransposons, and five retrotransposon fragments containing only portions of internal sequences and sometimes parts of LTRs [83,84]. Eight of nine families of LTR-retrotransposons are represented only by LTRs or LTR fragments, all full-length retrotransposons are members of the Tf2 family, and the five fragments are Tf1 or Tf2 sequences [83].…”

“…Tf1 is represented by 28 solo LTRs in the reference genome, compared to 35 solo LTRs for Tf2 [83]. Tf1 and Tf2 have similar sequences and are both members of the Ty3/gypsy superfamily of LTR retrotransposons [84,85]. However, Tf1 and Tf2 are both members of a distinct group of elements that uses self-priming to initiate reverse transcription [86].…”

“…As a result, studies of retrotransposition in S. pombe have focused on Tf1, though many studies of transcriptional regulation have focused on Tf2 elements in the reference genome. An excellent recent review addresses details of the steps of Tf1 retrotransposition and regulation of Tf1 and Tf2 by cellular factors [84], so only certain aspects of Tf1 and Tf2 retrotransposition and regulation will be highlighted in this review.…”

Yeast for virus research

“…For example, studies of yeast killer viruses have helped us to study cellular necrosis and apoptosis during virus-host interaction 14 15 16 17 , and to understand potential cellular viral restriction factors toward viral infections 18 19 . Since the integration process of yeast retrotransposons resembles in many ways retroviral integration, molecular studies of fission yeast Tf elements or budding yeast Ty elements provided insights into functions of retroviruses such as HIV or murine leukemia viruses 20 21 22 .…”

“…In addition, fission yeast has been used to carry out large-scale and functional characterization of small viral genomes such as human immunodeficiency virus type 1 (HIV-1) and Zika virus (ZIKV) 55 68 69 70 . General reviews on some of the related topics have been published previously 11 22 64 71 72 73 74 .…”