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This is just one option out of many, but what often works miracles in cases like this is to subtly alter the DNA oligo - add or remove 1-2 nt from the ends, or to methylate/phosphorylate/dephosphorylate etc. This is quite cheap for short oligos and therefore can be very cost effective. Artem -----Original Message----- From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Scott Berry Sent: Monday, August 11, 2008 1:57 AM To: [log in to unmask] Subject: [ccp4bb] Protein/DNA microcrystals Hello All, I am trying to crystallise a heterodimeric transcription factor complex bound to DNA. At the moment I have a construct which crystallises in many different conditions, all containing a PEG (3350, 4K, 6K, 8K) as precipitant along with various buffers and/or salts. When fine screening (in hanging drops) around these conditions I commonly get a shower of microcrystals at 20% (or higher) PEG. At lower PEG concentrations the crystals become more round and eventually the drops progress to phase separation (around 15% PEG). Lowering the protein concentration in the drops results in similar numbers of crystals forming, which end up smaller in size. I have tried seeding with the microcrystals from higher to lower PEG concentrations which results in crystal melting/phase separation. Also, the best crystals seem to grow on the glass slide and when I try to move them or pick them up, they are quite soft and hard to handle. Any advice on how to optimise conditions for growth of larger crystals and also tips for handling these soft crystals would be great. Thanks, Scott