The relative stabilities of specific embryonic mRNAs that persist in Drosophila melanogaster larvae were determined using an approach that combined RNA density labeling with cell-free translation. Unlike the other methods commonly used to measure the decay of individual mRNAs, the density labeling approach does not depend on the use of transcriptional inhibitors or on the measurement of precursor pool specific activities. Using this approach, we have determined that different embryonic mRNA species persist for varying periods during subsequent development, with half-lives ranging from approximately 2 to approximately 30 h. The embryonic histone mRNAs are relatively unstable; they are no longer detectable by 9 h of larval development. By 41 h of larval development, 90% of the nonhistone mRNAs assayed have decayed considerably; computerized scanning densitometry of translation products indicates that these transcripts are not decaying as members of discrete half-life classes. The persisting mRNAs that remain are very long-lived; their in vitro translation products can still be detected after 91 h of larval development. We have tentatively identified the mRNAs that encode actin, tropomyosin, and tubulin as members of this stable mRNA population. Although embryonic mRNAs do fall into these three broad classes of stability, they appear to decay with a continuum of half-lives. Because the range of half-lives is so great, mRNA stability is probably an important factor controlling mRNA abundance during Drosophila development.

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