3.2.2. Ectothiorhodospiraceae

According to GMT sequences, species of *Halorhodospira* and *Ectothiorhodospira* form two clearly separated groups (Figure 1). In *Ectothiorhodospira*, species including *Thioalkalivibrio nitratireducens* and related species, GMT and DMT genes form a cluster together with the gene encoding S-adenosylmethionine synthetase (methionine adenosyl transferase, MAT), which is essential for performance of the methylation by providing the methyl donor S-adenosylmethionine. In the *Halorhodospira* species, in addition, the genes encoding S-adenosyl homocysteinase (SAHase) and 5,10-methylene tetrahydrofolate reductase (MTHFR) are included in this gene cluster (Table 3). The methionine synthase (MS) that completes the methionine cycle is located at a different location within the genome. The coordinated action of these enzymes is expected to allow optimal performance of betaine biosynthesis by providing the essential methyl groups and removing the byproduct S-adenosylhomocysteine, which strongly inhibits the reaction [15]. The chemotrophic *Arhodomonas aquaeolei* and *Nitrococcus mobilis* (only GMT-DMT cluster) form a distinct subbranch within the *Ectothiorhodospiraceae*.

There is a curiosity with the presence of a second additional single GMT gene within a few species of *Chromatiales*, which is phylogenetically distinct from the genes commonly found in other phototrophic bacteria, except the two *Rhodovibrio* species (Figure 1). We refer to these genes as the B-type methyltransferases in betaine biosynthesis, compared to the "common" system. GMT sequences of this B-type group form three lineages of phototrophic Gammaproteobacteria, (i) marine and halophilic *Chromatiaceae* species (*Halochromatium salexigens, Halochromatium glycolicum*, *Rhabdochromatium marinum* and *Thioflavicoccus mobilis*), (ii) *Halorhodospira* species (*Halorhodospira halophila*, *Halorhodospira neutriphila*), and (iii) *Ectothiorhodospira* species (*Ectothiorhodospira mobilis*, *Ectothiorhodospira marismortui*, *Ectothiorhodospira marina*) (Figure 1). In contrast, the B-type GMT genes of *Rhodovibrio sodomensis* and *Rhodovibrio salinarum* are the only ones for biosynthesis of betaine in these bacteria that are unique among *Alphaproteobacteria*. While this gene is included in a functional gene cluster and is quite likely active in the betaine synthesis of the *Rhodovibrio* species (GMT-DMT-MAT-SAHase), its role in the *Chromatiales* is unclear and it might represent an evolutionary relict or a backup.
