Figure 7–55 A riboswitch that responds to guanine. (A) in this example from bacteria, the riboswitch controls expression of the purine biosynthetic genes. when guanine levels in cells are low, an elongating rnA polymerase transcribes the purine biosynthetic genes, and the enzymes needed for guanine synthesis are therefore expressed. (B) when guanine is abundant, it binds the riboswitch, causing it to undergo a conformational change that forces the rnA polymerase to terminate transcription (see figure 6–11). (C) Guanine (red) bound to the riboswitch. Only those nucleotides that form the guanine-binding pocket are shown. Many other riboswitches exist, including those that recognize S-adenosylmethionine, coenzyme B12, flavin mononucleotide, adenine, lysine, and glycine. (Adapted from M. Mandal and r.r. Breaker, Nat. Rev. Mol. Cell Biol. 5:451–463, 2004. with permission from Macmillan publishers ltd; and C.K. vanderpool and S. Gottesman, Mol. Microbiol. 54:1076–1089, 2004. with permission from Blackwell publishing.)
Figure 7–56 Five patterns of alternative RNA splicing. in each case, a single type of rnA transcript is spliced in two alternative ways to produce two distinct mrnAs (1 and 2). the dark blue boxes mark exon sequences 1 that are retained in both mrnAs. the light blue boxes mark possible exon 2 sequences that are included in only one of the mrnAs. the boxes are joined by red lines to indicate where intron sequences (yellow) are removed. (Adapted from h. Keren et al. Nat. Rev. Genet. 11:345–355, 2010. with permission from Macmillan publishers ltd.)
Riboswitches are perhaps the most economical examples of gene control devices, inasmuch as they bypass the need for regulatory proteins altogether. In the example shown in Figure 7–55, the riboswitch controls transcription elongation, but they can also regulate other steps in gene expression, as we shall see later 1 in this chapter. Clearly, highly sophisticated gene control devices can be made from short sequences of RNA, a fact that supports the hypothesis of an early “RNA world.” 1 Alternative rnA Splicing Can produce Different forms of a protein 2 from the Same Gene
As discussed in Chapter 6 (see Figure 6–26), RNA splicing shortens the transcripts of many eukaryotic genes by removing the intron sequences from the mRNA pre-1 cursor. We also saw that a cell can splice an RNA transcript differently and thereby 2 make different polypeptide chains from the same gene—a process called alternative RNA splicing (Figure 7–56). A substantial proportion of animal genes (estimated at 90% in humans) produce multiple proteins in this way.
When different splicing possibilities exist at several positions in the transcript, a single gene can produce dozens of different proteins. In one extreme case, a Drosophila gene may produce as many as 38,000 different proteins from a single gene through alternative splicing (Figure 7–57), although only a fraction of these forms have thus far been experimentally observed. Considering that the Drosophila genome has approximately 14,000 identified genes, it is clear that the protein complexity of an organism can greatly exceed the number of its genes. This example also illustrates the perils in equating gene number with an organism’s complexity. For example, alternative splicing is rare in single-celled budding yeasts one out of 38,016 possible splicing patterns