TRANSCRIPTIONAL PROPERTIES OF RNA-POLYMERASE-II WITHIN TRIPLET REPEAT-CONTAINING DNA FROM THE HUMAN MYOTONIC-DYSTROPHY AND FRAGILE-X LOCI

Expansion of a (CTG)(n) segment within the 3'-untranslated region of t he myotonic dystrophy protein kinase gene alters mRNA production. The inherent ability of RNA polymerase II to transcribe (CTG)(17-255) trac ts corresponding to DNA from normal, unstable, and affected individual s, and the normal (CGG)(54) fragile X repeat tract, was analyzed using a synchronized in vitro transcription system. Core RNA polymerase II transcribed all repeat units irrespective of repeat length or orientat ion. However, approximately 50% of polymerases transiently halted tran scription (with a half-life of approximately 10 +/- 1 s) within the fi rst and second CTG repeat unit and a more transient barrier to elongat ion was observed roughly centered within repeats 6-9. Transcription wi thin the remainder of the CTG tracts and within the CCG, CGG, and CAG tracts appeared uniform with average transcription rates of 170, 250, 300, and 410 nucleotides/min, respectively. These differences correlat ed with changes in the sequence-specific transient pausing pattern wit hin the CNG repeat tracts; individual incorporation rates were slower after incorporation of pyrimidine residues. Unexpectedly, approximatel y 4% of the run-off transcripts were, depending on the repeat sequence , either 15 or 18 nucleotides longer than expected, However, these pro ducts were not produced by transcriptional slippage within the repeat tract.