Bacteriophage Mu is a Transposon

Hybrid gene creatures exist that possess the characteristics both of transposable elements and some other genetic element. For example, bacteriophage Mu is both a virus and a transposon. (Note that we are not talking about a virus that carries a transposon inserted within its DNA—a frequent occurrence—but about a genetic element that behaves as both a virus and a transposon simultaneously). When Mu DNA enters E.

bacteriophage Mu A bacterial virus that replicates by transposition and causes mutations by insertion within host cell genes conjugative transposon A transposon that is also capable of transferring itself from one bacterial cell to another by conjugation

FIGURE 15.22 Retron RNA and RNA/DNA Hybrid

Retron RNA only contains the gene for reverse transcriptase and a large 5' untranslated region. This folds into a hairpin structure so that the 5' end meets the 3' end. Reverse transcriptase recognizes a specific guanine near the front of the hairpin and begins DNA synthesis here. The RNA is partly degraded, resulting in a strange RNA/DNA hybrid structure.

Non-translated region of retron RNA 5' 3'

Folding

Dna made by reverse transcriptase

Degrade most of rna

FIGURE 15.22 Retron RNA and RNA/DNA Hybrid

Retron RNA only contains the gene for reverse transcriptase and a large 5' untranslated region. This folds into a hairpin structure so that the 5' end meets the 3' end. Reverse transcriptase recognizes a specific guanine near the front of the hairpin and begins DNA synthesis here. The RNA is partly degraded, resulting in a strange RNA/DNA hybrid structure.

coli, its bacterial host, it integrates at random into the host chromosome by transposition (Fig. 15.23). In other words, the whole of the Mu genome is a transposon. If Mu inserts into the middle of a host gene this will be inactivated. Early investigators noticed that infection with this virus caused frequent mutations and therefore named it Mu for "mutator" phage.

Like many bacterial viruses, Mu may lie dormant as a prophage or go lytic (see Ch. 17). When Mu replicates it does so by uncontrolled replicative transposition, not by replicating as a virus. This results in multiple copies of Mu inserted into the host DNA and destroys so many host genes that it is inevitably lethal. Unlike other viruses, the DNA of Mu is never found replicating as an independent molecule, free of the chromosome. The segments of DNA packaged into the virus particle contain a whole Mu genome plus small stretches of host DNA at the ends. Thus even inside the virus particle, Mu DNA is still inserted into host DNA! When the virus DNA infects a new cell, the Mu genome transposes out of the tiny fragment of host DNA it brought with it. Thus Mu is a true transposon and its DNA is never free.

Infection

Transposition

FIGURE 15.23 Bacteriophage Mu is a Transposon

Bacteriophage Mu attaches to the E. coli cell and injects its DNA into the cytoplasm. Once inside, the Mu DNA inserts into the host chromosome via transposition. Notice that the flanking DNA from the previous host is not inserted. Once integrated, Mu DNA may undergo so many transpositions that cellular functions are destroyed. When the Mu DNA is packaged into virus particles, short lengths of host chromosome are also packaged attached to the Mu ends; therefore, Mu DNA is always integrated into host DNA.

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