All the following are false regarding C. diphteriae toxin EXCEPT:

Toxin production depends on optimum concentration of iron REF: Jawetz, Melnick, & Adelberg's Medical Microbiology, 24TH edition Chapter 13. DIPHTHERIA TOXIN: Exotoxin Heat-labile polypeptide (MW 62,000) Lethal in a dose of 0.1 ug/kg Two fragments: Fragment B (MW = 38,000) has no independent activity but is required for the transpo of fragment A into the cell. Fragment A inhibits polypeptide chain elongation. provided nicotinamide adenine dinucleotide (NAD) is present--by inactivating the elongation factor EF-2. In vitro production of this toxin depends largely on the concentration of iron. Toxin production is optimal at 0.14 lig of iron per milliliter of medium but is viually suppressed at 0.5 lig/mL. Other factors influencing the yield of toxin in vitro are osmotic pressure, amino acid concentration, pH, and availability of suitable carbon and nitrogen sources. The factors that control toxin production in vivo are not well understood.