CHAPTER 14 Regulation of Bacterial Gene Transcription
CHAPTER 15 RNA Polymerase Ⅱ: Basal Transcription
CHAPTER 16 RNA Polymerase Ⅱ: Regulation
CHAPTER 17 RNA Polymerase Ⅱ: Cotranscriptional and Posttranscriptional Processes
CHAPTER 18 Ribosomal RNA, Transfer RNA, and Organellar RNA Synthesis
SECTION 6 Protein Synthesis
CHAPTER 19 Protein Synthesis: The Genetic Code
CHAPTER 20 Protein Synthesis: The Ribosome
Index
精彩書摘
These nucleases, which were called restriction endonudeasesbecause they blocked or restricted viral replication, act only on DNAwith specific recognition sequences and only when the recognition sequences are not modified. Host DNA is protected because it has methylgroups attached to specific bases within the recognition sequence.
Three major types of restrictionmodification systems have beenstudied (Table S.2). Type I restrictionmodification systems consist offive polypeptide subunits: two identical restriction endonuclease subunits (R), two identical modification subunits (M), and a specificitysubunit (S). If the sequence that is recognized by the specificity subunitdoes not have a methyl group, then one of two things will happen. Themodification subunits will methylate the sequence and the DNA willbe protected, or the restriction subunits will cleave the DNA at a nonspecific site, often I kb or more from the recognition sequence, and theDNA will be degraded. Type II restrictionmodification systems aremade of two independent enzymes, a homodimeric restriction endonuclease and a monomeric methyl transferase (methylase). Type 1I restrictionmodification enzymes recognize sequences that are 4 to 8 bp long.Type II methylases transfer methyl groups to bases within the recognition sequence and type II endonucleases cleave DNA within the recognition sequence. Type III restrictionmodification systems consist oftwo subunits, a modification subunit and a restriction subunit. Modification occurs within the recognition sequence but cleavage takesplace about 25 bp away from this site. The discussion that follows islimited to the type II endonucleases because they are the only one of thethree types that has been widely used to manipulate DNA.