REM1, a New Type of Long Terminal Repeat Retrotransposon in Chlamydomonas reinhardtii

Abstract

A new long terminal repeat (LTR) retrotransposon, named REM1, has been identified in the green alga Chlamydomonas reinhardtii. It was found in low copy number, highly methylated, and with an inducible transpositional activity. This retrotransposon is phylogenetically related to Ty3-gypsy LTR retrotransposons and possesses new and unusual structural features. A regulatory module, ORF3p, is present in an inverse transcriptional orientation to that of the polyprotein and contains PHD-finger and chromodomains, which might confer specificity of the target site and are highly conserved in proteins involved in transcriptional regulation by chromatin remodeling. By using different wild-type and mutant strains, we show that CrREM1 was active with a strong transcriptional activity and amplified its copy number in strains that underwent foreign DNA integration and/or genetic crosses. However, integration of CrREM1 was restricted to these events even though the expression of its full-length transcripts remained highly activated. A regulatory mechanism of CrREM1 retrotransposition which would help to minimize its deleterious effects in the host genome is proposed.

ACKNOWLEDGMENTS

We thank M. I. Macías for technical support, C. Santos and I. Molina for secretarial assistance, and A. Galván for critical reading of the manuscript.

This work was supported by Ministerio de Ciencia y Tecnología, Spain (MCYT grant no. BMC2002-03325), Junta de Andalucía, Spain (Plan Andaluz de Investigación, CVI-0128), and the Vth Framework Program of the EU (HPRN-CT-2002-00247).