Figure 1

Taxonomic distribution of Sigma 54 and the associated Enhancer Binding Proteins. The phyla Proteobacteria and Firmicutes have been divided into the constituent classes. Between brackets, the number of genomes with Sigma-54 over the total number of sequenced genomes is listed followed by the average number of EBP⁵⁴s with a discrete standard deviation. The corresponding data can be found in additional file 1. The ordering of the phyla is based on conserved gene arrangement comparisons [162], a concatenated alignment of 22 single-copy conserved genes [163] and the analysis of conserved indels [164]. Gemmatimonas was placed according to [165], Nitrospira according to [166] and Thermatogae according to [167]. On the right, the cell morphology in terms of number of membranes (monoderm vs. diderm), presence of LPS (from [64]) and nature of the cell wall peptidoglycan (Gram-, Gram+ or other) is given. The majority of phyla represent diderms, except for Tenericutes, Firmicutes and Actinobacteria. Chloroflexi are probably also monoderm [168] and some have been shown to have a thick cell wall and stain Gram positive [169]. Deinococcus radiodurans has a complex Gram + like cell wall that includes outer membrane-like structure and the cell wall and outer membrane can be shared by multiple cells [170]. Dictyoglomus thermophilum is diderm but can grow in bundles or spherical bodies which are surrounded by a common outer membrane [171]. Finally, the Thermotoga have an outer sheath-like envelope ('toga') and an atypical thin cell wall [172].