Abstract
Efficient synthesis of many small abundant RNAs is achieved by the proficient recycling of RNA polymerase (pol) III and stable transcription complexes. Cellular Alu and related retroposons represent unusual pol III genes that are normally repressed but are activated by viral infection and other conditions. The core sequences of these elements contain pol III promoters but must rely on fortuitous downstream oligo(dT) tracts for terminator function. We show that a B1-Alu gene differs markedly from a classical pol III gene (tRNAiMet) in terminator sequence requirements. B1-Alu genes that differ only in terminator sequence context direct differential RNA 3' end formation. These genes are assembled into stable transcription complexes but differ in their ability to be recycled in the presence of the La transcription termination factor. La binds to the nascent RNA 3' UUUOH end motif that is generated by transcriptional termination within the pol III termination signal, oligo(dT). We found that the recycling efficiency of the B1-Alu genes is correlated with the ability of La to access the 3' end of the nascent transcript and protect it from 3'-5' exonucleolytic processing. These results illuminate a relationship between RNA 3' end formation and transcription termination, and La-mediated reinitiation by pol III.
Highlights
Efficient synthesis of many small abundant RNAs is achieved by the proficient recycling of RNA polymerase III and stable transcription complexes
While studies indicate that TFIIIB bound to the template is sufficient to direct reinitiation, other data suggest that proper termination may be required for efficient recycling, suggesting that termination and reinitiation may be mechanistically linked [10, 12]
This view is consistent with studies of the human La antigen, a protein that recognizes the 3Ј terminal RNA motif UUUOH and serves as a termination factor that can regulate the recycling of pol III transcription complexes in vitro [13,14,15,16]
Summary
Vol 273, No 40, Issue of October 2, pp. 26110 –26116, 1998 Printed in U.S.A. Terminator-specific Recycling of a B1-Alu Transcription Complex by RNA Polymerase III Is Mediated by the RNA Terminus-binding Protein La*. B1-Alu genes that differ only in terminator sequence context direct differential RNA 3 end formation These genes are assembled into stable transcription complexes but differ in their ability to be recycled in the presence of the La transcription termination factor. While studies indicate that TFIIIB bound to the template is sufficient to direct reinitiation, other data suggest that proper termination may be required for efficient recycling, suggesting that termination and reinitiation may be mechanistically linked [10, 12] This view is consistent with studies of the human La antigen, a protein that recognizes the 3Ј terminal RNA motif UUUOH and serves as a termination factor that can regulate the recycling of pol III transcription complexes in vitro [13,14,15,16]. The recycling efficiency of these B1-Alu transcription complexes is correlated with the ability of La to stably protect the 3Ј end of the nascent RNA from processing
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