Transcriptional control of RAD51 expression in the ciliate Tetrahymena thermophila.

The expression of Rad51p, a DNA repair protein that mediates homologous recombination, is induced by DNA damage and during both meiosis and exconjugant development in the ciliate Tetrahymena thermophila. To completely investigate the transcriptional regulation of Tetrahymena RAD51 expression, reporter genes consisting of the RAD51 5' non-translated sequence (5' NTS) positioned upstream of either the firefly luciferase or green fluorescent protein coding sequences have been targeted for recombination at the macronuclear btu1-1 (K350M) locus of T. thermophila strain CU522. Expression from RAD51-luciferase reporter constructs has been directly quantified from transformant whole cell lysates. Luciferase is induced to maximum levels in transformants harboring the full-length RAD51-luciferase reporter gene following exposure to DNA damaging UV irradiation. A series of truncations, deletions, insertions, substitutions and inversions of the RAD51 5' NTS have led to the identification of three distinct transcriptional promoter elements. The first of these sequence elements is required for basal levels of transcription. The second modulates expression in the absence of DNA damage, whereas the third ensures increased RAD51 transcription in response to DNA damage and during meiosis. Tetrahymena RAD51 is tightly regulated through these transcriptional elements to produce the appropriate expression during conjugation, and in response to DNA damage.