The Model of Amyloid Aggregation of Escherichia coli RNA Polymerase σ 70 Subunit Based on AFM Data and In Vitro Assays

To propose a model for recently described amyloid aggregation of E.coli RNA polymerase sigma (70) subunit, we have investigated the role of its N-terminal region. For this purpose, three mutant variants of protein with deletions Delta 1-73, Delta 1-100 and Delta 74-100 were constructed and studied in a series of in vitro assays and using atomic force microscopy (AFM). Specifically, all RNA polymerase holoenzymes, reconstituted with the use of mutant sigma subunits, have shown reduced affinity for promoter-containing DNA and reduced activity in run-off transcription experiments (compared to that of WT species), thus substantiating the modern concept on the modulatory role of N-terminus in formation of open complex and transcription initiation. The ability of mutant proteins to form amyloid-like structures has been investigated using AFM, which revealed the increased propensity of mutant proteins to form rodlike aggregates with the effect being more pronounced for the mutant with the deletion Delta 1-73 (10 fold increase). sigma (70) subunit aggregation ability has shown complex dependence on the ionic surrounding, which we explain by Debye screening effect and the change of the internal state of the protein. Basing on the obtained data, we propose the model of amyloid fibril formation by sigma (70) subunit, implying the involvement of N-terminal region according to the domain swapping mechanism.