Summary: A temperature-sensitive mutant (Comb M8) has been isolated which is defective in genetic transformation at the restrictive temperature (42 °C). The mutant was selected on the basis of its transforming properties at 42 and 28 °C on agar plates spread with DNA. During growth at 37 °C the mutant shows normal development of transformability (competence), whereas at 42 °C competence is very low or completely absent. At 42 °C the mutant is not transfectable with DNA from bacteriophage 029, but susceptible to normal infection with the complete bacteriophage, indicating that the ability to take up exogenous DNA is impaired at this temperature.

In addition to the loss of competence the mutant shows a changed growth pattern at the restrictive temperature: cultures grow initially at the same rate as cultures of the parental strain up to the late exponential phase of growth and then abruptly change over to a much lower growth rate, simultaneously producing shorter cells than the parental strain. From a genetic analysis evidence has been obtained suggesting that the two altered properties of the mutant, loss of competence and a changed growth pattern at 42 °C, are due to the same mutation.

When the mutant is shifted during growth from the restrictive to the permissive temperature, its subsequent pattern of competence development is similar to that displayed by the parental strain after such a shift. Likewise, the response to an opposite shift is also similar in both strains. Only when the mutant is grown continuously at 42 °C, including the overnight incubation period, is the mutant genotype expressed completely.

The results suggest that at 42 °C the mutant is blocked in a relatively late step in the development of competence.


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