The Fortran program package GNOM carries out SAS data treatment and interpretation for the disperse non‐interacting dilute systems of different types. A general operator method based on Tikhonov's regularization technique is used. Here a finite real‐space distribution function [the correlation function p(r) for monodisperse systems and size distribution function D(R) for polydisperse ones] is searched for to fit the experimental data in accordance with the experimental conditions and the error bounds. It is possible to require non‐negativity of the solution. The package GNOM is applicable for arbitrary experimental conditions (arbitrary weighting functions and form factors) and provides solutions stable to statistical noise and termination effects. The program makes it possible to treat together experimental curves from the same object measured under different experimental conditions and to choose the best model from the given set. Interactive and batch versions are available. The program is adapted to NORD, VAX and IBM‐PC computers. It possesses convenient graphics possibilities.

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Many medically relevant Gram-negative bacteria use the type III secretion system (T3SS) to translocate effector proteins into the host for their invasion and intracellular survival. A multiprotein complex located at the cytosolic interface of the T3SS is proposed to act as a sorting platform by selecting and targeting substrates for secretion through the system. However, the precise stoichiometry and 3D organization of the sorting platform components is unknown. Here we reconstitute soluble complexes of the Salmonella Typhimurium sorting platform proteins including the ATPase InvC, the regulator OrgB, the protein SpaO and a recently identified subunit SpaO C, which we show to be essential for the solubility of SpaO. We establish domaindomain interactions, determine for the first time the stoichiometry of each subunit within the complexes by native mass spectrometry and gain insight into their organization using small-angle X-ray scattering. Importantly, we find that in solution the assembly of SpaO/SpaO C /OrgB/InvC adopts an extended L-shaped conformation resembling the sorting platform pods seen in in situ cryo-electron tomography, proposing that this complex is the core building block that can be conceivably assembled into higher oligomers to form the T3SS sorting platform. The determined molecular arrangements of the soluble complexes of the sorting platform provide important insights into its architecture and assembly.

The problem of error propagation using indirect methods for small-angle scattering data treatment is considered. In these methods, the number of parameters to be determined is normally larger than the maximum number of independent parameters predicted by the Shannon sampling theorem and the solution has to be regularized. It is shown in model examples that evaluation of the error propagation via the covariance matrix can lead to significant overestimation of the propagated errors. The reason is that the procedure involves inversion of an ill-conditioned matrix. As an alternative, the Monte Carlo simulation procedure is recommended.

A program for three‐dimensional rendering and interactive model refinement, ASSA, is presented. The program permits the simultaneous display and manipulation of low‐resolution envelope models obtained from solution‐scattering data and the crystallographic or electron microscopic models in the Brookhaven Protein Data Bank format. ASSA is coupled with the shape‐determination program of Svergun, Volkov, Kozin, Stuhrmann, Barberato & Koch [J. Appl. Cryst. (1997). 30, 000–000], thus allowing one to monitor the shape determination process in real time. The program ASSA is written in C using the PHIGS+ standard for the three‐dimensional solid rendering and the XView and XLib libraries for the menu‐driven user interface. The system runs on a Sun SPARC ZX‐20 workstation; a simplified IBM‐PC version is also available.