The structure of the stalk surface layer of a brine pond microorganism: correlation averaging applied to a double layered lattice structure.

The surface layer of the stalk of a prosthecate halophilic microorganism is a periodic array (space group p3m1) comprised of electron dense trimers with a centre to centre spacing of 9.0 nm. The structure is reminiscent of E. coli porin. We have demonstrated that the method of correlation averaging can be effectively used to separate overlapping lattices, and that this results in a higher fidelity reconstruction, when compared to the established method of Fourier filtration. Two statistical methods are used to determine the resolution of the correlation average as a function of the number of 'windows' averaged. (i) The phase residual of spatial frequencies in Fourier space is computed between independently obtained subaverages, and (ii) a new method of Q factor analysis examines the cumulative vector sum in Fourier space as a function of the number of windows averaged. Both methods give a resolution of 1/2.1 nm-1.