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Molecular and Cellular Biology, July 2004, p. 6184-6193, Vol. 24, No. 14
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.14.6184-6193.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

mRNA Capping Enzyme Activity Is Coupled to an Early Transcription Elongation

Department of Biochemistry and Molecular Biology, College of Pharmacy, Sungkyunkwan University, Suwon, Kyonggi-do 440-746,¹ School of Medicine, Sungkyunkwan University, Suwon,² Department of Biochemistry and Molecular Biology, Cancer Research Institute, Seoul National University College of Medicine, Seoul, South Korea³

Received 9 February 2004/ Accepted 19 April 2004

One of the temperature-sensitive alleles of CEG1, a guanylyltransferase subunit of the Saccharomyces cerevisiae capping enzyme, showed 6-azauracil (6AU) sensitivity at the permissive growth temperature, which is a phenotype that is correlated with a transcription elongation defect. This temperature-sensitive allele, ceg1-63, has an impaired ability to induce PUR5 in response to 6AU treatment and diminished enzyme-GMP formation activity. However, this cellular and molecular defect is not primarily due to the preferential degradation of the transcript attributed to a lack of cap structure. Our data suggest that the guanylyltransferase subunit of the capping enzyme plays a role in transcription elongation as well as cap formation. First, in addition to the 6AU sensitivity, ceg1-63 is synthetically lethal with elongation-defective mutations in RNA polymerase II. Secondly, it produces a prolonged steady-state level of GAL1 mRNA after glucose shutoff. Third, it decreases the transcription read through a tandem array of promoter-proximal pause sites in an orientation-dependent manner. Taken together, we present direct evidence that suggests a role of capping enzyme in an early transcription. Capping enzyme ensures the early transcription checkpoint by capping of the nascent transcript in time and allowing it to extend further.

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