Mammals
Budding
yeast
Exon definition
Intron definition
Exon ISS
U2 65 35
U2AF
YNYURAY
UACUAAC
Y(n) YAG
U(n) YAG
30 nt
ESE ESS GU ISE Exon
Exon
U1
5 ′ splice site
5 ′ splice site
3 ′ splice site
3 ′ splice site
Branch site
Branch site
Poly(Y)
tract
Poly(U)
tract
GURAGU
Exon GUAUGU
U1 U2
Figure 7.1 Conserved sequence elements of mammalian and budding yeast pre-mRNAs. Exons (cylinders) are separated
by introns (lines). The consensus sequences in mammals and budding yeast at the 5′ splice site, branch site, and 3′ splice
site are as indicated. N is any nucleotide, R is purine, and Y is pyrimidine. Mammals contain a polypyrimidine-rich stretch;
S. cerevisiae contains a polyuridine-rich stretch. Both are located between the branch site and the 3′ splice site. In
mammals, cross-exon complexes are formed during early stages of spliceosome assembly, while in S. cerevisiae the introns
are defined. The spliceosomal snRNPs U1 and U2 (green) are shown interacting with the splice sites. Mammals additionally
have the U2 auxiliary factor (U2AF), U2AF65 and U2AF35 (green) interacting with the 3′ splice site. They also use auxiliary
regulatory elements that either enhance the splicing process, namely, exonic and intronic splicing enhancers (ESE and ISE,
dark green cylinders), or inhibit spliceosome assembly, such as exonic and intronic splicing silencers (ESS and ISS, red
cylinders). These elements are often bound by SR proteins and hnRNPs.