An Invertible Segment of Mu Phage
المؤلف:
Robert Schleif
المصدر:
Genetics and Molecular Biology
الجزء والصفحة:
2nd Edition , p544-545
2025-08-03
492
The split-ended heteroduplexes described in the preceding section are observed with DNA that is obtained from Mu-infected cells. Mu DNA obtained instead from an induced lysogen yields heteroduplexes containing not only the split end, but half of them also contain an internal single-stranded region of 3,000 base pairs that forms a bubble (Fig.1). This bubble results from the fact that the region, called G, is inverted in about half of the phage.
Fig1. Representation of an electron micrograph of a portion of a denatured and reannealed Mu phage DNA duplex showing the split ends and G loop.
The ability of Mu to invert G can be abolished by a small deletion at the end of G or by a point mutation in the adjacent region of the DNA called gin. This point mutation can be complemented, thereby proving that gin encodes a protein that participates in inversion of G. Curiously, phage P1 also contains the same G loop, and P1 can complement Mu gin mutants.
The G-loop segment codes for proteins that determine the host range of Mu. Phage induced from lysogens with G in the plus orientation are able to adsorb to Escherichia coli K-12, but not to Citrobacter freundii or Escherichia coli C. When G is in the minus orientation, the abilities of the resultant phage to adsorb to these strains are reversed. This phenomenon provides an explanation of the heteroduplexing experiments. Those Mu able to adsorb to Escherichia coli K-12 must be in the plus orientation. Since the rate of inversion is low, the G region of most of the phage in the resulting lysate will still be in the plus orientation, and heteroduplexes prepared from this DNA will not contain G bubbles. Lysogens are different. Although the phage originally entered cells with the G loop in the plus orientation, during the many generations of growth from a single lysogen, G can invert, and eventually about half the cells in the population will contain Mu with G in the plus orientation and half will contain Mu with G in the minus orientation. Induction of the phage in this cell population and preparation of DNA heteroduplexes will yield half the molecules with G bubbles.
The mutants with G stuck in one orientation because of the deletion at the end of G or the point mutation in gin permit a particularly simple demonstration of the infectivity properties. Mu lysogens with G stuck in the minus orientation yield phage particles, but these are not infective on Escherichia coli K-12. Mu lysogens containing G stuck in the plus orientation are fully infective when plated on Escherichia coli K-12.
Genetic analysis of the G region of the phage shows that two sets of genes are involved: S and U, and S’ and U’. S and U are expressed when G is in the plus orientation and S’ and U’ are expressed when G is in the minus orientation (Fig.2). The promoter for these genes and the initial portion of the S gene is contained in the α region adjacent to G. Thus, G contains a variable portion of S and S’ as well as the intact U and U’ genes. This is an interesting example of a situation in which a given DNA sequence can be used to specify different products.
Fig2. Structure of the G region of phage Mu. Transcription of the S and U or S’ and U’ genes initiates at a promoter in the α region. The gin gene product acts at the inverted repeats, IR, to invert the G region at a low rate.
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