Intervening Sequence Acquired by Lateral Gene Transfer in Tropheryma whipplei Results in 23S rRNA Fragmentation

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Abstract
Completion of Tropheryma whipplei genome sequencing may provide insights into the evolution of the molecular mechanisms underlying the pathogenicity of this microorganism. The first postgenomic application was the successful design of a comprehensive culture medium that allows axenic growth of this bacterium, which is particularly recalcitrant to cultivation. This achievement in turn permitted analysis of T. whipplei RNA without contaminating eukaryotic nucleic acids. To obtain high-quality RNA, several extraction methods were compared, but under all conditions tested an atypical profile was observed. By using a Northern blot assay we demonstrated that an insertion sequence previously described in T. whipplei 23S rRNA is in fact an intervening sequence excised during maturation. This cleavage could involve an RNase III identified in the genome of this microorganism. Among the bacteria with a 23S rRNA insertion sequence, T. whipplei is the only gram-positive microorganism. We present phylogenetic evidence that this mobile genetic element was acquired by lateral gene transfer from another enteric bacterium.

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