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

Abstract

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.