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In Figure 2A the authors are using a transcriptional pulse chase to trap mRNA degradation intermediates.
In Figure 2A the authors are using a transcriptional pulse chase to trap mRNA degradation intermediates. They use a galactose inducible promoter to initiate new mRNA synthesis in the presence of radiolabeled nucleotides (the pulse). After 10 minutes (when the mRNA synthesis is complete) they remove the radiolabeled nucleotides and monitor degradation products of the newly synthesized radiolabeled mRNA (the chase). The polyG sequence is inserted at the the BglII site at position 28 (B28pGM). The pC marks the poly C sequence in the probe. There are 2 polyC sequences one at position 28 which can hybridize with the poly G and another at an internal location which does not have a complementary polyG sequence to hybridize to.