分子生物学（张海红）11 章.pptVIP

Topic 4 Examples of transposable enements and their regulation Two types of regulation appear as recurring themes: Transposons control the number of their copies present in a given cell. Trnasposons control target site choice. Chapter 11 4-1 IS4-family transposons are compact elements with multiple mechanisms for copy number control Tn10 transposes via the cut-and paste mechanism, using the DNA hairpin strategy to cleave the nontransfered strands. Tn10 limits its copy number in any given cell by strategies that restrict its transposition frequency. One mechanism is the use of an antisense RNA to control the expression of the transposase gene. By this mechanism, cells that carry more copes of Tn10 will transcribe more of the antisense RNA, which in turn will limit expression of the transposase gene. The transposition frequency will, therefore, be very low in such a strain. Chapter 11 Tn10 Structure: Mechanism: cut-and-paste mechanism Chapter 11 Anti-sense RNA Antisense regulation of Tn10 expression Figure 11-28 Chapter 11 4-2 Tn10 transposition is coupled to cellular DNA replication Bacteria methylate their DNA at GATC sites and GATC sites are hemimethylated for a few minutes. It is during the brief period—when the Tn10 DNA is hemimethylated—that transposition is more likely to occur. Both RNA polymerase and transposase bind more tightly to the hemimethylated sequences than to their fully methylated versions. As a result, when the DNA is hemimethylated, the transposase gene is most efficiently expressed, and the transposase protein binds most efficiently to the DNA. Chapter 11 Transposition of Tn10 after passage of a replication fork. Figure 11-29 Chapter 11 4-3 Phage Mu is an extrmely robust transposon Robust transposon Transposition way is similar to retroviruses Carry two genes related with transpotion: MuA MuB Regulation: target immunity to avoid transposing into its own DNA 15kb Chapter 11 Figure 11-30 Early steps of Mu transposition