Positive-function assays for Asp-400 and Gln-404 substitution mutants. (A) In vivo Φ(narG-lacZ) expression in a narL+ strain. Activities from cultures grown in the absence and the presence of nitrate are indicated as open and solid bars, respectively. Data are averaged from three to five independent experiments. Error bars show SD values. The 100% activity value corresponds to about 1,200 Miller units. (B) In vitro transmitter autokinase activities. Phospho-MBP-NarX was measured by filter binding as described in Materials and Methods. The 100% activity value corresponds to about 2 pmol of phospho-MBP-NarX. Data are averaged from two independent experiments. ○, wild-type; ◆, Q404H; ▲, Q404A; ■, Q404N; ▼, Q404E; ▽, D400E; □, D400N.