Epigenetic silencing of a foreign gene in nuclear transformants of Chlamydomonas.

TLDR

The unstable expression of introduced genes poses a serious problem for the application of transgenic technology in plants. The authors propose that heterochromatic chromosomal domains may mediate transcriptional inactivation of foreign genes in Chlamydomonas. In Chlamydomonas transformants, the bacterial aadA gene is transcriptionally suppressed by a reversible epigenetic mechanism that is not linked to DNA methylation or major chromatin changes, shows intermediate expression states, and is enhanced at lower temperatures, resembling position effect variegation.

Abstract

The unstable expression of introduced genes poses a serious problem for the application of transgenic technology in plants. In transformants of the unicellular green alga Chlamydomonas reinhardtii, expression of a eubacterial aadA gene, conferring spectinomycin resistance, is transcriptionally suppressed by a reversible epigenetic mechanism(s). Variations in the size and frequency of colonies surviving on different concentrations of spectinomycin as well as the levels of transcriptional activity of the introduced transgene(s) suggest the existence of intermediate expression states in genetically identical cells. Gene silencing does not correlate with methylation of the integrated DNA and does not involve large alterations in its chromatin structure, as revealed by digestion with restriction endonucleases and DNase I. Transgene repression is enhanced by lower temperatures, similar to position effect variegation in Drosophila. By analogy to epigenetic phenomena in several eukaryotes, our results suggest a possible role for (hetero)chromatic chromosomal domains in transcriptional inactivation.