Bacteriophage Genetics Life cycle and mutants of phages

第五章 噬菌体的遗传分析 Bacteriophage Genetics

第一节 噬菌体的繁殖和突变型 Life cycle and mutants of phages

Genetic material in a phage

一、烈性噬菌体（ virulent phage） n Phage T 4: Structure and life cycle ◇ one of T-even phages ◇ intricate structure ◇ genetic material is DNA, linear, about 165 kb, encode for more than 150 genes

Life cycle

二、温和噬菌体（temperate phage) n Phage λ: Structure and life cycle ◇ The genome is 48514 bp ◇ linear double-stranded DNA, form a ring after the phage injected into the host cell through the sticky ends ◇ clusters of genes with related functions

n Life cycle: ◇ lytic cycle (裂解周期） ◇ lysogenic cycle（溶源周期）

Key concepts: ◇ lysogeny（溶源性） ◇ prophage（原噬菌体） 　◇ lysogenic bacteria or lysogen （溶源菌） n

Integration of λchromosome initiate the lysogenic cycle

Lysogenic bacterium are immune to further injection by the same phage

The λ repressor protein is responsible for maintaining lysogenic state and for immunity to superinfection

Events that damage DNA may induce prophages to enter the lytic cycle

n Zygotic induction (合子诱导) Hfr（λ）×F－（λ）

n The λ lytic cycle is interlocked with the lysogenic cycle

三、噬菌体突变型（phage mutants） n n Phages are so small that they are visible only under the electron microscope We cannot produce a visible colony by plating

The formation of a plaque ( a clear area)

phage phenotype n Plaque morphology（噬菌斑形态突变型 ） n Altered host range（宿主范围突变型 ） n Conditional lethal （条件致死突变型 ） ◇ temperature sensitive mutations （ts） （温度敏感突变） ◇ suppressor-sensitive mutations（sus） （抑制因子敏感突变 ）

Suppressor-sensitive mutations n Nonsense mutation（无义突变）: 琥珀型（amber）：UAG 赭石型（ocher）：UAA 乳白型（opal）：UGA Nonsense mutation is conditional lethal mutation, because the host has nonsense suppressor

n Nonsense suppressor The nonsense suppressors encoded by altered t. RNA genes can insert a specific amino acid at the position of stop codon. Nonsense mutation and nonsense suppressors a, b, c

第二节 噬菌体的重组作图 Recombination mapping in bacteriophages

一、T 2突变型的两点测验 Two-point cross of phage T 2 n h－ can infect two E. coli strains (B and B-2) h＋ can infect only E. coli B when infect a mixture of E. coli B and B-2, plaque of h－ is distinct, while plaque of h＋ is fuzzy. n r－ rapidly lyses cells, producing large plaques r＋ slowly lyses cells, producing small plaques

h－r＋ × h＋ r－ n n n Firstly, E. coli B is infected with both parental T 2 phages The progeny phage is then spread onto a bacterial lawn composed of a mixture of E. coli B and B-2 Four plaque types appear

h－ r＋: distinct, small h＋ r－: fuzzy, large h ＋ r＋: fuzzy, small h－r － : distinct, large

二、T 4突变型的三点测验 Three-point cross of phage T 4

负干扰（negative interference） n n n In phage cross, negative interference often occurs Genetic exchange between phage chromosomes will occurs before, during, and after replication Recombination is not restricted to exchange between two chromosomes----three or more may be involved simultaneously