Regulation of LTR retrotransposons in Schizosaccharomyces pombe

The genome of the fission yeast Schizosaccharomyces pombe is host to a family of long terminal repeat (LTR) retrotransposons called Tf2, which is comprised of 13 full length elements and around 250 solo LTRs. The expression of these elements is subjected to chromatin based silencing during normal growth conditions but is induced in response to hypoxia via the action of Sre1 , a homologue of human SREBPtranscription factor. The aim of this study was to dissect the mechanisms that regulate Tf2 expression and to determine how they contribute to the control of element mobilisation. Characterisation of the Tf2 LTR element identified sequences that mediate its transcriptional control, while further studies identified roles for the Spt6 and Asf1 histone chaperones in Tf2 element silencing. Furthermore, similar transcription controls were shown operate at the related Tf1 LTR retrotransposons that are present in the genomes of other wild type s. pombe strains. To analyse the effects of transcriptional regulation upon Tf2 element propagation, an assay was established to measure the mobilisation frequency of a marked endogenous Tf2 element (Tf2-12natAI). Using this system the affect of key transcriptional regulators upon Tf2 mobilisation frequency was determined. The HIRA nucleosome assembly complex has previously been shown to be required for Tf2 silencing and accordingly element expression was found to be increased >10-fold in the absence of HIRA. Despite this finding only a marginal (1.8-fold) increase in the frequency of Tf2 mobilisation was observed. In contrast, expression of a constitutively active form of the transcription factor Sre1 (Sre1-N) resulted in a large increase in mobilisation (>10-fold) despite inducing Tf2 expression to less than 50% of that observed in the absence of HIRA. Furthermore, loss of the CENP-B homologue, Abp1 resulted in only a small increase in Tf2 expression (3-fold) but a significant increase in mobilisation (4-fold). Therefore, mobilisation frequency is not necessarily correlated with expression levels, suggesting that Tf2 mobilisation is subjected to additional controls. These studies also revealed that the Asf1 histone chaperone restricts Tf2 element propagation, while components of the RNAi pathway promote Tf2 mobilisation. In an effort to identify novel regulators of Tf2 expression, the homologues of the s. cerevisae bromodomain containing AAA-ATPase Yta7 were characterised. s. pombe contains two Yta7 homologues (SPAC31G5.19and SPBP22H7.05c)which were named Yta71 and Yta72 respectively. Yta71, but not Yta72, was found to restrict the expression of Tf2 and Tfi elements and their solo LTRs. Deletion of yta71+ was also found to result in sensitivity to the spindle poison thiabendazole and an increased level of chromosome loss. Consistent with these findings Yta71 is required for the integrity of centromeric heterochromatin and also the heterochromatin associated with the mating type (mat) locus. Thus Yta71 is required for transcriptional silencing at multiple loci in fission yeast.