8.5
Bactériophages à ADN
Les bactériophages, ou phages, sont des virus qui infectent spécifiquement les bactéries, utilisant leur matériel génétique pour détourner la machiner…
Les bactériophages à ADN sont des virus qui se présentent sous diverses formes et infectent les bactéries en injectant leur ADN.
Ils peuvent avoir soit de l’ADN simple brin appelé ADNsb, soit de l’ADN double brin appelé génomes ADNdb.
Par exemple, le phage ΦX174, un phage icosaédrique de l’ADNss, a un petit génome avec des gènes qui se chevauchent. Cela permet à plusieurs protéines d’être codées à travers différents cadres de lecture.
Ce phage se réplique par réplication en cercle roulant, formant un intermédiaire double brin avant de générer de nouveaux génomes.
Enfin, la descendance est libérée par lyse de l’hôte.
En revanche, M13, un phage filamenteux de l’ADNsb, sort de la cellule hôte par extrusion plutôt que par lyse, ce qui permet une production continue de phages tout en maintenant l’hôte en vie.
Parmi les phages d’ADNdb, le phage T7 se réplique de manière bidirectionnelle, formant des concatémères traités par une enzyme terminase pour l’emballage du génome.
D’autre part, Mu initie la réplication par transposition, en s’insérant dans plusieurs sites du génome hôte. Il excise ensuite et emballe son génome linéaire en nouveaux virions.
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Q1: What is the difference between ssDNA and dsDNA bacteriophages?
ssDNA bacteriophages contain single-stranded DNA genomes, while dsDNA bacteriophages contain double-stranded DNA. ssDNA phages like ΦX174 have compact genomes with overlapping genes that maximize genetic efficiency. dsDNA phages like T7 and Mu employ more elaborate replication mechanisms and generally have larger genomes, enabling complex interactions with bacterial hosts.
Q2: How does ΦX174 phage maximize its genetic efficiency?
ΦX174 uses overlapping genes where different reading frames encode multiple proteins from the same DNA sequence. This strategy allows the icosahedral ssDNA phage to pack more genetic information into its small genome. The phage replicates via rolling circle replication, forming a double-stranded intermediate before generating new genomes that are released through host lysis.
Q3: Why does M13 phage exit the host cell differently than ΦX174?
M13, a filamentous ssDNA phage, exits through extrusion rather than lysis, allowing continuous phage production while keeping the host alive. This contrasts with ΦX174, which lyses the host to release progeny. M13's sustained production strategy is widely used in molecular biology techniques such as phage display and recombinant DNA technologies.
Q4: What role does the terminase enzyme play in T7 phage replication?
T7 replicates bidirectionally, forming concatemers—long continuous strands containing multiple genome units. The terminase enzyme cleaves these concatemers into individual genome-sized segments for encapsulation into new phage particles. This efficient replication strategy has made T7 a model system in synthetic biology and genetic engineering.
Q5: How does Mu phage replicate differently from other dsDNA bacteriophages?
Mu replicates through transposition rather than conventional rolling circle or bidirectional replication. It integrates into various sites within the bacterial genome and undergoes multiple rounds of transposition before excising itself for packaging into new virions. Mu's transposition duplicates host DNA at insertion sites, often disrupting genes, making it useful for mutagenesis and bacterial genome mapping.
Q6: What is rolling circle replication in bacteriophages?
Rolling circle replication is a mechanism used by ssDNA phages like ΦX174 to generate new viral genomes. The process begins by synthesizing a complementary strand, forming a double-stranded DNA intermediate called the replicative form. This RF serves as a template for continuous synthesis of new viral genomes that are then packaged into virions.
Q7: How do bacteriophages inject their DNA into bacterial hosts?
DNA bacteriophages infect bacteria by injecting their genetic material directly into the host cell. Once inside the bacterial cytoplasm, the phage DNA hijacks the host's cellular machinery for replication. Different phage types then employ distinct replication strategies—such as rolling circle replication in ssDNA phages or bidirectional replication in dsDNA phages—to generate progeny.