Optimization of a ribosomal structural domain by natural selection
Optimization of a ribosomal structural domain by natural selection
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DOI：
10.1021/bi052544p
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发表时间：
2006-05-30
期刊：
影响因子：
2.9
通讯作者：
Draper, David E.
中科院分区：
文献类型：
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作者：
Maeder, Corina;Conn, Graeme L.;Draper, David E.
大核糖体亚基中的保守、独立折叠结构域由 58 nt 的 rRNA 和单个蛋白质 L11 组成。携带大肠杆菌序列的 rRNA 片段的三级结构在体外稍微稳定，但可以通过在其他生物体中发现的突变而基本上稳定。为了区分自然选择没有在大肠杆菌中进化出更稳定的 rRNA 结构的可能原因，评估了影响 rRNA 三级结构的突变对 rRNA 稳定性和 L11 亲和力（在 rRNA 片段的背景下）的体外影响或对细胞生长速率和核糖体 L11 含量的体内影响。相对于野生型序列，rRNA 片段稳定性范围为 -4 至 +9 kcal/mol。 -4 至 +5 kcal/mol 范围内的变异在体内几乎没有可观察到的影响，而更多的不稳定突变（> 7 kcal/mol）则无法耐受。数据表明，复合物的体内稳定性约为-6 kcal/mol，并且只要维持复合物的最低稳定性，任何单一的三级相互作用对于功能来说都是可有可无的。根据这些数据，该领域的演变似乎并未受到固有的结构或功能稳定性限制。估计的稳定性对应于在任何时间从 L11 分离的每个细菌细胞仅几个核糖体；因此，进一步提高稳定性的选择优势可能很小，以至于被其他竞争性选择压力所抵消。
A conserved, independently folding domain in the large ribosomal subunit consists of 58 nt of rRNA and a single protein, L11. The tertiary structure of an rRNA fragment carrying the Escherichia coli sequence is marginally stable in vitro but can be substantially stabilized by mutations found in other organisms. To distinguish between possible reasons why natural selection has not evolved a more stable rRNA structure in E. coli, mutations affecting the rRNA tertiary structure were assessed for their in vitro effects on rRNA stability and L11 affinity (in the context of an rRNA fragment) or in vivo effects on cell growth rate and L11 content of ribosomes. The rRNA fragment stabilities ranged from -4 to +9 kcal/ mol relative to the wild-type sequence. Variants in the range of -4 to +5 kcal/mol had almost no observable effect in vivo, while more destabilizing mutations (> 7 kcal/mol) were not tolerated. The data suggest that the in vivo stability of the complex is roughly -6 kcal/mol and that any single tertiary interaction is dispensable for function as long as a minimum stability of the complex is maintained. On the basis of these data, it seems that the evolution of this domain has not been constrained by inherent structural or functional limits on stability. The estimated stability corresponds to only a few ribosomes per bacterial cell dissociated from L11 at any time; thus the selective advantage for any further increase in stability may be so small as to be outweighed by other competing selective pressures.