1. Maintain strains for growth on minimal plates, using maltose as the carbon source and supplemented with the appropriate auxotrophic requirements. ER-1647 requires histidine, methionine, and tryptophane.
2. Grow a single colony overnight in an appropriate volume of minimal medium with the required supplements.
3. Spin down the overnight cells and resuspend in 0.5 vol of 10 mM MgSO4. The cells are stable for about a month when resuspended in 10 mM MgSO4, but only a few days if not. Of course, fresh cells work better.
4. Use 200-300 |L of cells for a 100-mm plate or 600-800 for a 150-mm plate. In vitro packaging mixes contain a large excess of phage tails, so use the larger volume for them. Maltose-grown cells also help a lot.
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