Page 61 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN006H-655 June 29, 2001 21:21

510 Gene Expression, Regulation of

FIGURE 7 A simplistic model for spliceosome assembly. (a) The 5 splice site and the branch site are defined via a
direct base pairing between the RNA components of the U1 and U2 snRNPs and the precursor-RNA, respectively.
(b) Efficient recruitment of the U snRNPs to the spliceosome is aided by non-snRNP proteins. Thus, SR proteins

facilitate U1 snRNP binding to the 5 splice site, whereas U2AF binds to the polypyrimidine tract at the 3 splice
site and helps U2 snRNP binding to the branch site. The U4/U6–U5 triple snRNP is recruited to form the mature

spliceosome. SR proteins bring the 5 and 3 splice sites in close proximity for the catalytic steps of splicing by making

simultaneous contact with splicing factors binding to the 5 and 3 splice sites, respectively (see also Fig. 8).
U2, U4, U5, and U6), ranging in size from 107 to 210 shared among different U snRNPs, whereas other snRNP
nucleotides, have been shown to participate in splicing. proteins are unique to each U snRNP. During spliceosome
In vivo the snRNAs are found complexed to 6–10 pro- assembly the ends of the introns are in part identified by
teins, generating the so-called small nuclear ribonucle- RNA–RNA base pairing between the precursor-RNA and

oprotein particles (snRNPs). Some snRNP proteins are a U snRNP (Fig. 7). For example, the 5 splice site is