Nongenic, bidirectional transcription precedes and may promote developmental DNA deletion in Tetrahymena thermophila

A large number of DNA segments are excised from the chromosomes of the soma tic nucleus during development of Tetrahymena thermophila. How these germli ne-limited sequences are recognized and excised is still poorly understood. We have found that many of these noncoding DNAs are transcribed during nuc lear development. Transcription of the germline-limited M element occurs fr om both DNA strands and results in heterogeneous transcripts of < 200 b to > 1 kb. Transcripts are most abundant when developing micro- and macronucle i begin their differentiation. Transcription is normally restricted to unre arranged DNA of micronuclei and/or developing nuclei, but germline-limited DNAs can induce their own transcription when placed into somatic macronucle i. Brief actinomycin D treatment of conjugating cells blocked M-element exc ision, providing evidence that transcription is important for efficient DNA rearrangement. We propose that transcription targets these germline-limite d sequences for elimination by altering chromatin to ensure their accessibi lity to the excision machinery.