Phylogenetic studies of newly isolated freshwater Magnetospirilla using cbb and mam genes

The phylogeny and general relatedness of prokaryotes is determined by comparisons of the sequences of rRNA genes, most commonly the 16S rRNA gene. Comparisons between other gene sequences have been used for this purpose and some have supported conclusions from 16S rRNA genes while others have not. In this study, 13 new magnetospirilla were phylogenetically characterized using the sequences of the 16S rRNA gene as well as the genes for forms I and II ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) (cbbL and cbbM, respectively) and for two magnetosome membrane proteins unique to magnetotactic bacteria, mamJ and mamK. Polymerase chain reaction (PCR) with degenerate primers designed for the specific genes under study was used to amplify a large portion of the genes. PCR products were cloned and sequenced and used for the construction of phylogenetic trees. Based on 16S rRNA gene sequences, the magnetospirilla phylogenetically span, more as a continuum rather than as clearly delineated groups, over two genera based on the current accepted sequence divergence between organisms for genera (>5%). While almost all strains appear to fit into the genus Magnetospirillum, strain LM-1 appears to represent a new genus. Phylogeny of these strains based on cbbM sequences was reasonably consistent with that from 16S rRNA genes. The cbbL gene was not a good choice for this study as most strains did not possess this gene. Relatedness and phylogeny of the strains based on mamJ and mamK sequences was more complex. Although our data set is not complete, some specific strains shown to be closely related by 16S rRNA gene sequence, also appeared to be closely related based on one or both of the mam gene sequences (e.g., strains UT-1, LM-2 and M. gryphiswaldense strain MSR-1). Other strains did not show this type of relationship. Because of these somewhat inconsistent results, those from mam gene sequences might reflect evolution of the magnetosome gene island (MAI) in magnetospirilla rather than relatedness between strains.