What is SOB containing 20 mM MgSO4 ? - 20mM MgSO4 only or 20mM MgSO4 + 10 mM MgCl2 (Dec/20/2010 )

Hello,

I am preparing SOB media containing 20 mM MgSO4. I read several protocols that make me confuse.

Some recipe is MgCl2 (10 mL/L of 1M MgCl2) and MgSO4 (20 mL/L of 1M MgSO4).
Another is only MgSO4 (20 mL/L of 1M MgSO4).
But some is MgCl2 (10 mL/L of 1M MgCl2) and MgSO4 (10 mL/L of 1M MgSO4).

If the SOB media will be used for preparing competent cells, which recipe is the correct one.

I do appreciate your kind assistance in advance.
Thank you very much
Urai

SOB (super-optimal broth) with 20 mM Mg²⁺, composed of 10 mM MgCl₂ and 10 mM MgSO₄ is the correct formulation. Read Hanahan 1983, J. Mol Biol 166; 557-580 for more information.

I think it is the total concentration of Mg²⁺ which is important, there are various different recepies using different salts to give the 20mM Mg²⁺ but they should all give similar results.