Practice question

Nonsense-mediated mRNA decay NMD is surveillance pathway eliminating transcripts harboring premature termination codons PTC to prevent synthesis of truncated proteins with potentially dominant-negative toxic effects. During splicing exon junction complex EJC deposited 20-24 nucleotides upstream of exon-exon borders. Normal termination in last exon lacks downstream EJC; PTC located more than 50-55 nucleotides upstream recruits Upf1 RNA helicase, Upf2, Upf3 after translation termination interacting with terminating ribosome and eRF1 eRF3. SMG1 kinase phosphorylates Upf1 triggering SMG6 endonuclease and SMG5-SMG7 mediated decapping deadenylation recruiting exonucleolytic decay. NMD also regulates normal transcripts with long 3' UTRs and upstream ORFs controlling gene expression.