A novel taxonomic marker that discriminates between morphologically complex actinomycetes

In the era where large whole genome bacterial data sets are generated routinely, rapid and accurate molecular systematics is becoming increasingly important. However, 16S ribosomal RNA sequencing does not always offer sufficient resolution to discriminate between closely related genera. The SsgA-like proteins (SALPs) are developmental regulatory proteins in sporulating actinomycete, whereby SsgB actively recruits FtsZ during sporulation-specific cell division. Here we present a novel method to classify actinomycetes, based on the extraordinary way the SsgA and SsgB proteins are conserved. The almost complete conservation of the SsgB amino acid sequence between members of the same genus, and its high divergence even between closely related genera, provides high quality data for the classification of morphologically complex actinomycetes. Our analysis validates Kitasatospora as a sister genus to Streptomyces in the family Streptomycetaceae and suggests that Micromonospora, Salinispora and Verrucosispora may represent different clades of the same genus. It is also apparent that the amino-acid sequence of SsgA is an accurate determinant for the ability of streptomycetes to produce submerged spores, dividing the phylogenetic tree of streptomycetes into LSp (liquid culture sporulation) and NLSp (no liquid culture sporulation) branches. A new phylogenetic tree of industrially relevant actinomycetes is presented and compared to that based on 16S rRNA sequences