The Genome Reduction Excludes the Ribosomal Rescue System in Acholeplasmataceae

The trans-translation process is a ribosomal rescue system for stalled ribosomes processing truncated mRNA. The genes ssrA and smpB fulfil the key functions in most bacteria, but some species have either lost these genes or the function of the ribosomal rescue system is taken over by other genes. To date, the ribosomal rescue system has not been analysed in detail for the Acholeplasmataceae. This family, in the Mollicutes class, comprises the genus Acholeplasma and the provisional taxon "Candidatus Phytoplasma". Despite their monophyletic origin, the two clades can be separated by traits such as not representing primary pathogens for acholeplasmas versus being phytopathogenic for the majority of phytoplasmas. Both taxa share reduced genomes, but only phytoplasma genomes are characterised by a remarkable level of instability and reduction. Despite the general relevance of the ribosomal rescue system, information is lacking on coding, the genomic context and pseudogenisation of smpB and ssrA and their possible application as a phylogenetic marker. Herein, we provide a comprehensive analysis of the ribosomal rescue system in members of Acholeplasmataceae. The examined Acholeplasmataceae genomes encode a ribosomal rescue system, which depends on tmRNA encoded by ssrA acting in combination with its binding protein SmpB. Conserved gene synteny is evident for smpB, while ssrA shows a less conserved genomic context. Analysis of the tmRNA sequences highlights the variability of proteolysis tag sequences and short conserved sites at the 5'- and 3'-ends. Analyses of smpB provided no hints regarding the coding of pseudogenes, but they did suggest its application as a phylogenetic marker of Acholeplasmataceae - in accordance with 16S rDNA topology. Sequence variability of smpB provides sufficient information for species assignment and phylogenetic analysis.