The transcription Initiation site for rat 45S precursor ribosomal RNA synthesis was determined by nuclease protection mapping with two single-strand endonucleases, SI and mung bean, and one single-strand exonuclease, ExoVII. These experiments were performed with end-labeled ribosomal DNA from double-stranded pBR322 recombinants and from single-stranded M13 recombinants. Results from experiments using both kinds of DNA and all three enzymes showed that the 5' end of 45S RNA mapped to a unique site 125 bases upstream from the H1nd III site 1n the ribosoraal DNA gene. The DNA surrounding this site (designated +1) was sequenced from -281 to +641. The entire sequence of this region shows extensive homology to the comparable region of mouse. This Includes three stretches of T residues 1n the non-coding strand between +300 and +630. Two sets of direct repeats adjacent to these T-r1ch regions are observed. Comparison of the mouse and human ribosomal DNA transcription Initiation sites with the rat sequence reported 1n this paper demonstrates a conserved sequence at +2 to +16, CTGACACGCTGTCCT. This suggests that this region may be Important for the Initiation of transcription on mammalian ribosomal UNAs.
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