MBE Advance Access originally published online on August 4, 2006
Molecular Biology and Evolution 2006 23(11):2112-2122; doi:10.1093/molbev/msl082
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© 2006 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Research Articles |
Impact of the Excision of an Ancient Repeat Insertion on Rickettsia conorii Guanylate Kinase Activity
* Information Génomique & Structurale, CNRS UPR 2589, IBSM, Marseille cedex, France
Unité des Rickettsies, Faculté de Médecine, Université de la Méditerranée, CNRS UMR 6020, IFR 48, Marseille cedex, France
E-mail: chantal.abergel{at}igs.cnrs-mrs.fr.
The genomic sequencing of Rickettsia conorii revealed a new family of Rickettsia-specific palindromic elements (RPEs) capable of in-frame insertion in preexisting open reading frames (ORFs). Many of these altered ORFs correspond to proteins with well-characterized or essential functions in other microorganisms. Previous experiments indicated that RPE-containing genes are normally transcribed and that no excision of the repeat occurs at the mRNA level. Using mass spectrometry, we now confirmed the retention of the RPE-derived amino acid residues in 4 proteins successfully expressed in Escherichia coli, raising the general question of the consequences of this common insertion event on the fitness of Rickettsia enzymes. The predicted guanylate kinase activity of the R. conorii gmk gene product was measured both on the RPE-containing and RPE-excised recombinant proteins. We show that the 2 proteins are active but exhibit substantial differences in their affinity for adenosine triphosphate, guanosine monophosphate, and catalytic constants. The distribution of the RPEgmk insert among Rickettsia species indicates that the insertion event is ancient and occurred after the divergence of Rickettsia felis and R. conorii but before that of Rickettsia helvetica and R. conorii. We found no evidence that the gmk gene fixed adaptive changes to compensate the RPE peptide insertion. Furthermore, the analysis of the rates of divergence in 23 RPE-containing genes indicates that coding RPE repeats tend to evolve under weak selective constraint, at a rate similar to intergenic noncoding RPE sequences. Altogether, these results suggest that the insertion of RPE-encoded "selfish peptides," although respecting the original fold and activity of the host proteins, might be slightly detrimental to the enzyme efficiency within limits tolerable for slow-growing intracellular parasites such as Rickettsia.
Key Words: Rickettsia conorii • palindromic coding repeat • guanylate kinase • enzyme fitness • recombinant protein expression
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