The σ factor of DB1005 contains two amino acid substitutions (I198A and I202A) in the promoter -10 binding region. It has been confirmed that this σ factor is responsible for the temperature sensitivity of DB1005. An investigation was conducted into how the mutantσ could cause temperature-sensitive (Ts) cell growth by analysing its structural stability, cellular concentration and transcriptional activity. The mutant σ was unstable even at the permissive temperature of 37°C (t 59 min), whereas the wild-type counterpart was fairly stable under the same conditions (t > 600 min). However, neither wild-type σ nor mutant σ was stable at 49°C (t 34 min and 23 min, respectively). Analyses of the rates of σ synthesis revealed that DB1005 was able to compensate for unstable σ by elevating the level of σ at 37°C but not at 49°C. Moreover, overexpression of the mutant σ at 49°C could not suppress the Ts phenotype of DB1005. This indicates that the temperature sensitivity of DB1005 is not due to insufficient σ concentration in the cell. The greater decline of an already reduced activity of the mutant σ at 49°C suggests that the temperature sensitivity of DB1005 is instead the result of a very low activity of σ probably below a critical level necessary for cell growth.


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