MESSENGER-RNA DESTABILIZATION TRIGGERED BY PREMATURE TRANSLATIONAL TERMINATION DEPENDS ON AT LEAST 3 CIS-ACTING SEQUENCE ELEMENTS AND ONE TRANS-ACTING FACTOR

Nonsense mutations in a gene can accelerate the decay rate of the mRNA transcribed from that gene, a phenomenon we describe as nonsense-medi ated mRNA decay. Using amber (UAG) mutants of the yeast PGK1 gene as a model system, we find that nonsense-mediated mRNA decay is position d ependent, that is, nonsense mutations within the initial two-thirds of the PGK1-coding region accelerate the decay rate of the PGK1 transcri pt less-than-or-equal-to 12-fold, whereas nonsense mutations within th e carboxy-terminal third of the coding region have no effect on mRNA d ecay. Moreover, we find that this position effect reflects (1) a requi rement for sequences 3' to the nonsense mutation that may be necessary for translational reinitiation or pausing, and (2) the presence of an additional sequence that, when translated, inactivates the nonsense-m ediated mRNA decay pathway. This stabilizing element is positioned wit hin the coding region such that it constitutes the boundary between no nsense mutations that do or do not affect mRNA decay. Rapid decay of P GK1 nonsense-containing transcripts is also dependent on the status of the UPF1 gene. Regardless of the position of an amber codon in the PG K1 gene, deletion of the UPF1 gene restores wild-type decay rates to n onsense-containing PGK1 transcripts.