Alternative conformers of 5S ribosomal RNA and their biological relevance

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Abstract
Different conformational states of Escherichia coli 5S ribosomal RNA that may participate in protein biosynthesis were either detected experimentally or predicted on the basis of phylogenetic sequence comparisons. The A conformer exists in a high-salt form (AH) that binds ribosomal proteins and assembles into the 50S subunit and in a low-salt form (AL), of uncertain biological relevance, that binds at least 1 ribosomal protein and differs in tertiary structure from the AH form. Experimentally, the AH form was investigated comprehensively and the AL form partially. There is also a B conformer that exhibitsan altered secondary structure and does not assemble with ribosomal proteins. For this conformer to be functionally active, it must be both discrete and universal among 5S RNAs. Its structure was examined by employing single and double strand specific ribonucleases and nucleotide-specific chemical reagents. The B form exhibits a secondary structure only a part of which is both universal and conformationally homogeneous, and the whole B form cannot participate in protein biosynthesis. Progressive structural changes occur during the transitions AH .fwdarw. AL .fwdarw. B and provide evidence that the structural alteration during the transition AH .fwdarw. AL may be universal, which reinforces the view that the AL form is of biological relevance.