第三部分chapter23

Figure 23.11 An intron codes for an endonuclease that makes a doublestrand break in DNA. The sequence of the intron is duplicated and then inserted at the break.
GGGAGG5’
Figure 23.10 Reactions catalyzed by RNA have the same features as those catalyzed by proteins, although the rate is slower. The KM gives the concentration of substrate required for half-maximum velocity; this is an inverse measure of the affinity of the enzyme for substrate. The turnover number gives the number of substrate molecules transformed in unit time by a single catalytic site.
The gene has a group I intron that contains a coding for endonucleases sequence. ω+: some strains of yeast with intron but absent in others (ω). Genetic crosses between ω+ and ω - are polar: the progeny are usually ω+.
Figure 23.7 The position of the IGS in the tertiary structure changes when P1 is formed by substrate binding.
Figure 23.3 The excised intron can form circles by using either of two internal sites for reaction with the 5' end, and can reopen the circles by reaction with water or oligonucleotides.
Figure 23.3 The excised intron can form circles by using either of two internal sites for reaction with the 5' end, and can reopen the circles by reaction with water or oligonucleotides.
23.2 Group I introns undertake selfsplicing by transesterification
Figure 23.1 Splicing of the Tetrahymena 35S rRNA precursor can be followed by gel electrophoresis. The removal of the intron is revealed by the appearance of a rapidly moving small band. When the intron becomes circular, it electrophoreses more slowly, as seen by a higher band.

Figure 23.11 An intron codes for an endonuclease that makes a doublestrand break in DNA. The sequence of the intron is duplicated and then inserted at the break.
23.4 Ribozymes have various catalytic activities
Figure 23.6 Excision of the group I intron in Tetrahymena rRNA occurs by successive reactions between the occupants of the guanosine-binding site and substrate-binding site. The left exon is red, and the right exon is purple.
Chapter 23
Catalytic RNA
23.1 Introduction
The idea that only proteins have enzymatic activity was deeply rooted in biochemistry. The enzyme ribonuclease P is a ribonucleoprotein that contains a single RNA molecule bound to a protein. Small RNAs of the virusoid class have the ability to perform a self-cleavage reaction. Introns of the group I and group II classes possess the ability to splice themselves out of the pre-mRNA that contains them.
The common theme of these reactions is that the RNA can perform an intramolecular or intermolecular reaction that involves cleavage or joining of phosphodiester bonds in vitro. RNA splicing is not the only means by which changes can be introduced in the informational content of RNA.
23.5 Some introns code for proteins that sponsor mobility
Three types of protein functions have been identified in open reading frames of these introns. They are concerned with either DNA or RNA metabolism: endonucleases, which cleave at target sites in DNA to allow the insertion of intron sequences; reverse transcriptases, which are involved in generating DNA copies of the intronic RNA; maturases, which are needed for splicing out the particular intron from the pre-mRNA.
Figure 23.9 Catalytic reactions of the ribozyme involve transesterifications between a group in the substrate-binding site and a group in the Gbinding site.
Figure 16.18 Retrotransposition of non-LTR elements occurs by nicking the target to provide a primer for cDNA synthesis on an RNA template.
Figure 23.12 Reverse transcriptase coded by an intron allows a copy of the RNA to be inserted at a target site generated by a doublestrand break.
Figure 23.5 Placing the Tetrahymena intron within the β-galactosidase coding sequence creates an assay for self-splicing in E. coli. Synthesis of β-galactosidase can be tested by adding a compound that is turned blue by the enzyme. The sequence is carried by a bacteriophage, so the presence of blue plaques indicates successful splicing.
Figure 23.2 Selfsplicing occurs by transesterification reactions in which bonds are exchanged directly. The bonds that have been generated at each stage are indicated by the shaded boxes.
23.3 Group I introns form a characteristic secondary structure