Structure determination of the O-methyltransferase NovP using the `free lunch algorithm' as implemented in SHELXE

NovP is an S-adenosyl-L-methionine-dependent O-methyltransferase from Streptomyces spheroides (subunit MW = 29 967 Da). Recombinant N-terminally His-tagged NovP crystallizes in space group P2, with approximate unit-cell parameters a = 51.81, b = 46.04, c = 61.22 Å, β = 105.0°, giving a solvent content of 44% for a single copy of the His-tagged protomer per asymmetric unit. Native synchrotron data to a resolution of 1.35 Å were combined with three other native data sets collected at lower resolution (both in-house and at the synchrotron) for the sake of completeness and better scaling. Data to 2.45 Å resolution were subsequently recorded in-house from a single mercury derivative. Three partial mercury sites could be located with SHELXD, but the resulting phases had a mean error of about 81° and in our hands did not yield an interpretable map using standard automated software. Nevertheless, the structure of NovP could be solved by first tracing a small part of the structure by hand and then extrapolating within and beyond the experimental resolution limit using the `free lunch algorithm' in SHELXE. The resulting phases have a mean phase error of 17° relative to a refined model.