Abstract
While Fourier-Bessel methods gave rise to the first three-dimensional reconstruction of an object from electron microscopic images, and these methods have dominated three-dimensional reconstruction of helical filaments and tubes for 30 years, single-particle approaches to helical reconstruction have emerged within the past 10 years that are now the main method being used. The Iterative Helical Real Space Reconstruction (IHRSR) approach has been the main methodology, and it surmounts many of the problems posed by real polymers that are flexible, display less than crystalline order, or are weakly scattering. The main difficulty in applying this method, or even Fourier-Bessel methods, is in determining the approximate helical symmetry. This chapter focuses on some of the intrinsic ambiguities that are present when trying to determine the helical symmetry from power spectra of images and argues that complementary techniques or some form of prior knowledge about the subunit may be needed to have confidence in the solution that is found.