Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    F factor ] traPc traO traJ MQHi I transfer proteins t t Î tra genes f¡ + R factor finO t I traP H 0 H tra genes Figure 3.14: model for the effect of traO mutations in the sex factor. A mutation in traO which prevents recognition of the F-transfer inhibitor allows the adjacent gene traJ to be expressed and the traJ product then turns on the transfer genes. Since traJ is plasmid specific, the R factor remains repressed. The traO mutation is eis -dominant, that is it acts upon only the traJ gene adjacent to it. Since, however, traO mutants are dominant in causing transfer of all sex factors present in the cell, the result of expression of traJ is to switch on the transfer genes and cause transfer of any sex factor. groups FI, Ico and X, however, are able to inhibit the transfer of traO~ and finP~ sex factors; no mutation that releases F from this inhibition has yet been identified, and so the mechanism(s) of this inhibition remain un known. Within this class there is a difference in that an Ico plasmid prevents surface exclusion but X and FI plasmids do not; the implication is that the Ico fi + mechanism switches off the entire transfer operon of the sex factor, but the X and FI mechanisms leave at least the traS gene able to function. An FI plasmid is distinct in being the only factor that inhibits transfer but allows the sex factor to produce sex pili; all the other fi + plasmids prevent the production of pili. Another mechanism is identified by the R62 plasmid of the la compatibility group: this inhibits the transfer of traO~ and finP~ sex factors, but a mutation in the F factor designated traQ~ allows the sex factor to transfer (Willetts and Paranchych, 1974). Since the finOlfinP system acts to prevent expression of traJ, it should be unable to inhibit the transfer of traJ~ mutants; all the plasmids displaying other systems of fertility inhibition, however, were able to inhibit transfer of traJ~ sex factors (to allow transfer to occur at all, these plasmids also carry a further mutation conferring partial expression of the transfer