Na phosphate buffer

T. reesei RUT-C30 (ATCC 56765) was purchased from ATCC (Manassas, VA) and T. atroviride Ta13 (MUT 6701) was isolated from
wheat seeds (Algeria) and deposited at the Mycotheca Universitatis
Taurinensis (MUT, Turin, Italy).
2,6-Dimethoxyphenol, 3,5-dinitrosalicylic
acid, 4-nitrophenyl butyrate, acetonitrile, bovine serum albumin,
carboxymethyl cellulose, citrate solution, citric acid, formic acid,
malic acid, methanol, Na acetate buffer, Na phosphate buffer, Na phosphate–citrate,
potato dextrose agar, trichloroacetic acid, Tris-HCl buffer, and Triton
X-100 were purchased from Merck (Darmstadt, Germany).

Coomassie Blue dye, Na-phosphate buffer, and paraformaldehyde were purchased from Merck (Steinheim, Germany). Karnovsky’s fixative, OsO₄, and hexamethyldisiloxane (HMDSO) were bought from SPI Supplies (West Chester, PA, USA). Milli-Q water was used to prepare solutions. Stainless-steel foil (50 μm) AISI 321 was obtained from Goodfellow (Hamburg, Germany). Carbon HB Pencil was bought from Staedtler (Nuremberg, Germany). All other chemicals and reagents were purchased from Sigma-Aldrich (Munich, Germany) unless otherwise mentioned.

The aim of these experiments was to evaluate the effect of different L-arabinose concentrations on the expression of target genes involved in the BCAA biosynthetic pathway, which are under the control of an araBAD promoter in the mutant E. coli strains engineered in this study.
Cultures were prepared by inoculating 50 μL of a cryostock of the corresponding E. coli strain into 15 mL tubes pre-loaded with 5 mL of 1:3 diluted supplemented mineral salt medium containing 5 g L⁻¹ glucose (Merck, Darmstadt, Germany), 0.1 M Na-phosphate buffer (Merck, Darmstadt, Germany) and 100 µg mL⁻¹ ampicillin (Sigma-Aldrich, Munich, Germany). For the tunable E.coli strains, medium also contained 25 µg mL⁻¹ chloramphenicol (Sigma-Aldrich, Munich, Germany). Different concentrations of L-arabinose were added to the different cultures prepared for each strain and these were incubated at 37 °C and 250 rpm, overnight. OD₆₀₀ was then measured after a cultivation time of 16 h. Results are reported in Additional file 1: Figure S3.