Teste pipoca (não vale colar!)

Teste pipoca (não vale colar!)

Paul Nelson

Para fazer o teste — ou brincar com ele — eis aqui a sua informação de background, e não vale colar olhando em livros-texto de biologia celular ou usar as bases de dados do Google ou PubMed.

1. Topoisomerases são enzimas essenciais encontradas em todos os organismos, que solucionam problemas topológicos que surgem da estrutura de dupla hélice do DNA.
Bem, vamos presumir que:

2. Todos os organismos na Terra compartilham de um ancestral comum, o LUCA (Last Universal Common Ancestor – Último Ancestral Comum Universal).

OK — hora de fazer o teste. Considerando-se (1) e (2), o que nós deveríamos esperar encontrar no que diz respeito à homologia das topoisomerases quando nós examinamos os organismos que descenderam do LUCA em partes diferentes da Árvore da Vida?

Múltipla escolha:

A) Topoisomerases homólogas

B) Topoisomerases não-homólogas

C) Eu não posso dizer.

Faça suas melhores tentativas, e depois vá aqui.

[Este artigo é Open Access.]

Não perca o comentário dos autores, 12 linhas abaixo. Na coluna do lado direito, na primeira página.

Concorda ou discorda?

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Nucleic Acids Research Advance Access published online on February 9, 2009
Nucleic Acids Research, doi:10.1093/nar/gkp032

Survey and Summary

Phylogenomics of DNA topoisomerases: their origin and putative roles in the emergence of modern organisms

Patrick Forterre and Danièle Gadelle*

Institut de Génétique et Microbiologie, Univ Paris-Sud, 91405 Orsay Cedex, France CNRS UMR 8621 and Institut Pasteur, 75015 Paris, France

*To whom correspondence should be addressed. Tel: +0033169156445; Fax: +0033169157808; Email: daniele.gadelle@igmors.u-psud.fr

Received October 31, 2008. Revised January 12, 2009. Accepted January 12, 2009.

ABSTRACT

Topoisomerases are essential enzymes that solve topological problems arising from the double-helical structure of DNA. As a consequence, one should have naively expected to find homologous topoisomerases in all cellular organisms, dating back to their last common ancestor. However, as observed for other enzymes working with DNA, this is not the case. Phylogenomics analysesindicate that different sets of topoisomerases were present in the most recent common ancestors of each of the three cellular domains of life (some of them being common to two or three domains), whereas other topoisomerases families or subfamilies were acquired in a particular domain, or even a particular lineage, by horizontal gene transfers. Interestingly, two groups of viruses encode topoisomerases that are only distantly related to their cellular counterparts. To explain these observations, we suggest that topoisomerases originated in an ancestral virosphere, and that various subfamilies were later on transferred independently to different ancient cellular lineages. We also proposed that topoisomerases have played a critical role in the origin of modern genomes and in the emergence of the three cellular domains.

INTRODUCTION

DNA topoisomerases (hereafter referred to as topoisomerases) are molecular magicians that are absolutely essential to solve topological problems arising from the double-helical structure of DNA (1,2). Type I topoisomerases (Topo I) introduce transient DNA single-stranded breaks in order to change the topological linking number of a double-stranded DNA molecule by steps ofone, whereas type II topoisomerases (Topo II) introduce transient double-stranded breaks and change the linking number by steps of two. Although topological problems are obvious with circular DNA genomes, they also occur with linear DNA, as indicated by the requirement of topoisomerases in eukaryotes, and in viruses with long linear genomes (from around 100 kb to 1 Mb). From such consideration, one should have naively expected to find in all cells two homologous topoisomerases descendant from two ancestral topoisomerases present in the Last Universal Common Ancestor (LUCA), which was usually assumed to contain a DNA genome (3). An intelligent designer would have probably invented only one ubiquitous Topo I and one ubiquitous Topo II to facilitate the task of future biochemists. The reality turned out to be quite different, and more interesting. As in the case of otherenzymes working with DNA, such as DNA polymerases, the distribution of topoisomerases families and sub-families among modern organisms is not congruent with the universal tree of life based on 16S rRNA sequence comparison (with the trinity Archaea, Bacteria and Eukarya). This is a challenging observation, since the phylogenies of many other important cellular proteins (universal ribosomal proteins, large RNA polymerase subunits, components of the protein-secretion system, ATP syntheses), as well as whole genome phylogenies (based on various methods) follow the tripartite rule.

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