Yeast Ty1 Retrotransposition Is Stimulated by a Synergistic Interaction between Mutations in Chromatin Assembly Factor I and Histone Regulatory Proteins

ABSTRACT

A screen for host mutations which increase the rate of transposition of Ty1 and Ty2 into a chromosomal target was used to identify factors influencing retroelement transposition. The fortuitous presence of a mutation in the CAC3 gene in the strain in which this screen was undertaken enabled us to discover that double mutaions of cac3 and hir3, but neither of the two single mutations, caused a dramatic increase in the rate of retrotransposition. We further showed that this effect was not due to an increase in the overall level of Ty1 mRNA. Two subtle cac3 phenotypes, slight methyl methanesulfonate (MMS) sensitivity and reduction of telomeric silencing, were significantly enhanced in the cac3 hir3 double mutant. In addition, the growth rate of the double mutant was reduced. HIR3 belongs to a class ofHIR genes that regulate the transcription of histones, while Cac3p, together with Cac1p and Cac2p, forms chromatin assembly factor I. Other combinations of mutations in cac and hir genes (cac3 hir1, cac3 hir2, and cac2 hir3) also increase Ty transposition and MMS sensitivity and reduce the growth rate. A model explaining the synergistic interaction between cac and hirmutations in terms of alterations in chromatin structure is proposed.

ACKNOWLEDGMENTS

We are grateful to P. D. Kaufman and M. A. Osley for strains and plasmids and for sharing unpublished data. We also thank L. Prakash, D. Schild, K. Struhl, R. Gietz, M. Rose, and E. Alani for plasmids, strains, and libraries and I. Derkatch and S. Prakash for helpful comments on the manuscript. We are indebted to Jack Gibbons for help with the immunogold electron microscopy, to M. Gerami-Nejad for assistance with immunofluorescence microscopy, to A. Kagan for help with the mutant screen, and to the employees of the University of Florida sequencing facility.

This work was supported by National Institutes of Health (NIH) grants GM50365 to S.W.L. and GM38626 to J.B.