PREMATURE TERMINATION OF TUBULIN GENE-TRANSCRIPTION IN XENOPUS-OOCYTES IS DUE TO PROMOTER-DEPENDENT DISRUPTION OF ELONGATION

We have shown previously that the Xenopus alpha-tubulin gene, XalphaT1 4, exhibits premature termination of transcription when injected into oocyte nuclei- The 3' ends of prematurely terminated transcripts are f ormed immediately downstream of a stem-loop sequence found in the firs t 41 bp of the 5' leader. We show here, using deleted constructs, that premature termination requires the presence only of sequences from -2 00 to +19 relative to the initiation site. Deletion of the stem-loop d oes not increase the production of extended transcripts, and premature termination apparently continues at nonspecific sites. This finding i ndicates that disruption of the elongation phase of transcription rath er than abrogation of a specific antitermination mechanism is the caus e of premature termination in XalphaT14. We also found that disruption of elongation on a reporter gene could be induced specifically by com petition with XalphaT14 promoters. To identify which elements of the p romoter might interact with elongation determinants to cause this comp etition, we constructed a series of internal promoter mutants. Most mu tations in the -200 to -60 region of the promoter had some effect on i nitiation frequency but did not cause any significant change in levels of premature termination. However, mutations in the core promoter tha t removed the TATA box consensus caused major changes in initiation an d resulted in a marked decrease in the production of prematurely termi nated transcripts relative to extended transcripts. We discuss why suc h promoters can apparently escape the disruption of elongation that le ads to premature termination.