A repeated 82 base pair sequence in genomic DNA of the rat was previously proposed as being a control element governing brain (neuron) specific genetic expression. This intronic sequence, termed the brain 'identifier' (ID), is complementary to small RNA species localized in brain cytoplasm, and it was thought to be represented specifically in RNA produced by brain nuclei in vitro. The RNA blot analyses of total nuclear and polyadenylated heterogeneous nuclear RNA described in the present report show that this ID sequence is also present in the liver and kidney in abundances similar to those in the brain. This repeated sequence is not, therefore, restricted to transcripts produced in the brain as suggested from previous transcriptional 'runoff' experiments. Measurements on rat and mouse nuclear RNA indicate that the abundance of ID sequence transcript is roughly proportional to the number of copies of this repeat in the respective genomes. This suggests a rather random genomic location and transcription of this sequence. From these results it seems improbable that the ID sequence functions as a transcriptional level control element in genes expressed specifically in the brain.
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