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Representatives of the colorless sulfur bacteria of the genus Beggiatoa use reduced sulfur compounds in the processes of lithotrophic growth, which is accompanied by the storage of intracellular sulfur. However, it is still unknown how the transformation of intracellular sulfur occurs in Beggiatoa representatives. Annotation of the genome of Beggiatoa leptomitoformis D-402 did not identify any genes for the oxidation or reduction of elemental sulfur. By searching BLASTP, two putative persulfide dioxygenase (PDO) homologs were found in the genome of B. leptomitoformis . In some heterotrophic prokaryotes, PDO is involved in the oxidation of sulfane sulfur. According to HPLC-MS/MS, the revealed protein was reliably detected in a culture sample grown only in the presence of endogenous sulfur and CO 2 . The recombinant protein from B. leptomitoformis was active in the presence of glutathione persulfide. The crystal structure of recombinant PDO exhibited consistency with known structures of type I PDO. Thus, it was shown that B. leptomitoformis uses PDO to oxidize endogenous sulfur. Additionally, on the basis of HPLC-MS/MS, RT-qPCR, and the study of PDO reaction products, we predicted the interrelation of PDO and Sox-system function in the oxidation of endogenous sulfur in B. leptomitoformis and the connection of this process with energy metabolism.

A giant multidomain protein of striated and smooth vertebrate muscles, titin, consists of tandems of immunoglobulin (Ig)- and fibronectin type III (FnIII)-like domains representing β-sandwiches, as well as of disordered segments. Chicken smooth muscles express several titin isoforms of ~500-1500 kDa. Using various structural-analysis methods, we investigated in vitro nonspecific amyloid aggregation of the high-molecular-weight isoform of chicken smooth-muscle titin (SMT HMW , ~1500 kDa). As confirmed by X-ray diffraction analysis, under near-physiological conditions, the protein formed amorphous amyloid aggregates with a quaternary cross-β structure within a relatively short time (~60 min). As shown by circular dichroism and Fourier-transform infrared spectroscopy, the quaternary cross-β structure-unlike other amyloidogenic proteins-formed without changes in the SMT HMW secondary structure. SMT HMW aggregates partially disaggregated upon increasing the ionic strength above the physiological level. Based on the data obtained, it is not the complete protein but its particular domains/segments that are likely involved in the formation of intermolecular interactions during SMT HMW amyloid aggregation. The discovered properties of titin position this protein as an object of interest for studying amyloid aggregation in vitro and expanding our views of the fundamentals of amyloidogenesis.