Maeder Corina, Conn Graeme L, Draper David E
Program in Molecular and Computational Biophysics and Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Biochemistry. 2006 May 30;45(21):6635-43. doi: 10.1021/bi052544p.
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.
大核糖体亚基中一个保守的、可独立折叠的结构域由58个核苷酸的rRNA和单一蛋白质L11组成。携带大肠杆菌序列的rRNA片段的三级结构在体外稳定性较差，但可通过其他生物体中发现的突变得到显著稳定。为了区分自然选择为何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片段的稳定性范围为-4至+9千卡/摩尔。-4至+5千卡/摩尔范围内的变体在体内几乎没有可观察到的影响，而更具去稳定作用的突变（>7千卡/摩尔）则无法耐受。数据表明，该复合物在体内的稳定性约为-6千卡/摩尔，并且只要维持复合物的最低稳定性，任何单个三级相互作用对于功能都是可有可无的。基于这些数据，该结构域的进化似乎并未受到稳定性方面固有的结构或功能限制的约束。估计的稳定性相当于在任何时候每个细菌细胞中只有少数核糖体与L11解离；因此，稳定性进一步提高的选择优势可能非常小，以至于被其他竞争性选择压力所抵消。
