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Microbiology 154 (2008), 1482-1490; DOI  10.1099/mic.0.2007/015545-0
© 2008 Society for General Microbiology

Internuclear gene silencing in Phytophthora infestans is established through chromatin remodelling

Pieter van West1, Samantha J. Shepherd1, Claire A. Walker1, Shuang Li1, Alex A. Appiah1, Laura J. Grenville-Briggs1, Francine Govers2 and Neil A. R. Gow1

1 Aberdeen Oomycete Group, College of Life Sciences and Medicine, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK
2 Laboratory of Phytopathology, Binnenhaven 5, 6709 PD Wageningen, The Netherlands

Correspondence
Pieter van West
p.vanwest{at}abdn.ac.uk

In the plant pathogen Phytophthora infestans, nuclear integration of inf1 transgenic DNA sequences results in internuclear gene silencing of inf1. Although silencing is regulated at the transcriptional level, it also affects transcription from other nuclei within heterokaryotic cells of the mycelium. Here we report experiments exploring the mechanism of internuclear gene silencing in P. infestans. The DNA methylation inhibitor 5-azacytidine induced reversion of the inf1-silenced state. Also, the histone deacetylase inhibitor trichostatin-A was able to reverse inf1 silencing. inf1-expression levels returned to the silenced state when the inhibitors were removed except in non-transgenic inf1-silenced strains that were generated via internuclear gene silencing, where inf1 expression was restored permanently. Therefore, inf1-transgenic sequences are required to maintain the silenced state. Prolonged culture of non-transgenic inf1-silenced strains resulted in gradual reactivation of inf1 gene expression. Nuclease digestion of inf1-silenced and non-silenced nuclei showed that inf1 sequences in silenced nuclei were less rapidly degraded than non-silenced inf1 sequences. Bisulfite sequencing of the endogenous inf1 locus did not result in detection of any cytosine methylation. Our findings suggest that the inf1-silenced state is based on chromatin remodelling.

Abbreviations: 5-AC, 5-azacytidine; BuA, butyric acid; HDAC, histone deacetylase; PTGS, post-transcriptional gene silencing; TGS, transcriptional gene silencing; TSA, trichostatin A






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