| Summary: | <p>Abstract</p> <p>Background</p> <p>Along the chromosome of the obligate intracellular bacteria <it>Protochlamydia amoebophila </it>UWE25, we recently described a genomic island Pam100G. It contains a <it>tra </it>unit likely involved in conjugative DNA transfer and <it>lgrE</it>, a 5.6-kb gene similar to five others of <it>P. amoebophila</it>:<it> lgrA </it>to <it>lgrD, lgrF</it>. We describe here the structure, regulation and evolution of these proteins termed LGRs since encoded by "Large G+C-Rich" genes.</p> <p>Results</p> <p>No homologs to the whole protein sequence of LGRs were found in other organisms. Phylogenetic analyses suggest that serial duplications producing the six LGRs occurred relatively recently and nucleotide usage analyses show that <it>lgrB, lgrE </it>and <it>lgrF </it>were relocated on the chromosome. The C-terminal part of LGRs is homologous to Leucine-Rich Repeats domains (LRRs). Defined by a cumulative alignment score, the 5 to 18 concatenated octacosapeptidic (28-meric) LRRs of LGRs present all a predicted α-helix conformation. Their closest homologs are the 28-residue RI-like LRRs of mammalian NODs and the 24-meres of some <it>Ralstonia </it>and <it>Legionella </it>proteins. Interestingly, <it>lgrE</it>, which is present on Pam100G like the <it>tra </it>operon, exhibits Pfam domains related to DNA metabolism.</p> <p>Conclusion</p> <p>Comparison of the LRRs, enable us to propose a parsimonious evolutionary scenario of these domains driven by adjacent concatenations of LRRs. Our model established on bacterial LRRs can be challenged in eucaryotic proteins carrying less conserved LRRs, such as NOD proteins and Toll-like receptors.</p>
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