Key Concepts of Section 10.2

• Because of alternative splicing of primary transcripts, the use of alternative promoters, and cleavage at different poly(A) sites, different mRNAs may be expressed from the same gene in different cell types or at different developmental stages (see Figure 10-18).

• Alternative splicing can be regulated by RNA-binding proteins that bind to specific sequences near regulated splice sites. Splicing repressors may sterically block the binding of splicing factors to specific sites in pre-mRNAs or inhibit their function. Splicing activators enhance splicing by interacting with splicing factors, thus promoting their association with a regulated splice site. The RNA sequences bound by splicing repressors are called intronic or exonic splicing silencers, depending on their location in an intron or exon. RNA sequences bound by splicing activators are called intronic or exonic splicing enhancers.

• In RNA editing, the nucleotide sequence of a pre-mRNA is altered in the nucleus. In vertebrates, this process is relatively rare, and only single-base C to U changes have been observed, but those changes can have important consequences by altering the amino acid encoded by an edited codon (see Figure 10-22).