Figure 2. Over-expression of σ⁵⁴ enables E. coli to heterologously produce oxytetracycline.

(a) The enzymatic pathway responsible for the biosynthesis of oxytetracycline. (b). ESI-LC-MS/MS analysis of an oxytetracycline standard and the organic extracts from E. coli cultures containing the oxytetracycline gene cluster and over-expressing σ⁵⁴, σ^S, or FecI. These traces show the ion extraction data from the Q3 scan of MS/MS experiments. Q1 was used to select the [MH]⁺ ion for oxytetracycline (m/z = 461). Ion extractions were performed for the oxytetracycline fragment m/z = 283 from the Q3 scan. Signals with a peak width of less than 0.1 s were regarded as noise and removed with the noise filter application. (c). The MS² spectrum of the m/z = 461 peak for the oxytetracycline authentic standard and heterologously produced oxytetracycline. See also Figure S3.