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We have identified a number of mutations that alter E.coli's ability to utilize lactose and to regulate ß-galactosidase and lactose permease expression. Now our task is determine how many different genes these mutations represent. As you may remember (see MORGAN and MULLER) they used genetic mapping to answer this type of question. We can do genetic crosses in bacteria, but it involves a rather different type of sexual interaction. |
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Sex in bacteriaIn E. coli, sex, such as it is, is determined by the 'fertility factor'. The fertility factor is a plasmid, known as the F-plasmid. We can think of bacteria that harbor an F-plasmid as being of one sex and those without it as the other. Learn a little more about plasmids. The discovery of bacterial sexuality was made by Joshua Lederberg and Edward Tatum in 1947 (both won the Nobel Prize in 1958).Lederberg was an assistant professor at age 22. |
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A typical E. coli has hundreds of "common" or "type 1" pili on its surface -- these are invisible with a light microscope. Each pilus is a protein tube. Pili can be binding sites for bacterial viruses, which use the pilus's central channel to inject their genetic material into the cell. Pili are immotile and shorter than the flagella that are responsible for motility. |
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The F-plasmid contains the genes required to build a special type of pili, known as an F-pilus. F-pili are ~8 nm in diameter, and have an ~2 nm central channel (about the diameter of a naked strand of double-stranded DNA). There are typically 1 to 4 F-pili per cell. I f an F+ cell encounters an cell that does not have the F-plasmid (an F- cell), the two cells undergo a process known as conjugation -- the F-pili forms a bridge between the two cells.A single-stranded break is made in the double-stranded DNA of the F-plasmid. |
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DNA replication begins; a newly synthesized DNA strand is transported from the F+ to the F- cell through the F-pilus. Once transferred, the plasmid DNA is replicated and the F- cell becomes F+. What made conjugation so important was the observation that occasionally genes from host chromosome are transferred along with the F-plasmid. |
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Conjugation was made into a practical genetic tool by the discovery by William Hayes and Luca Cavalli-Sforza that there are strains of E. coli in which host genes were transferred up to 10,000 times more frequently. These Hfr (high frequency) strains are formed by the random integration of the F-plasmid into the host chromosome. In the chromosome. The integrated F-plasmid is still able to induce the formation of F-pili and conjugation. |
revised 9 July 2003 |
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