FIGURE 5.

Possible polar effects on downstream gene expression due to manipulation of TAS operon. (A) TAS operons are negatively autoregulated operons which do not encode a terminator. In the wild type, the downstream gene is cotranscribed with TA genes and its expression is considered optimal. (B) If TAS along with promoter are deleted, there is no expression of the downstream gene due to lack of a promoter. (C) If the toxin also is deleted, there could be over expression of the downstream gene due to missing corepressor, the toxin. (D) If the toxin and antitoxin are deleted but not the promoter, there could be high overexpression of the downstream gene due to lack of repression. (E) If a mutation is induced in the toxin gene such that its product is not toxic but can act as a corepressor (Overgaard et al., 2008), the expression of the downstream gene is likely to be identical to that of the wild type, an ideal case to study the function of TAS.