Ribosomal RNA Interactions

Ribosomal RNA Interactions

	Ribosomal RNA Interactions - An account of the structure and synthesis of ribosomal RNA has already been given in the chapter on RNA. It has been noted that the 70S prokaryote ribosome contains 168 rRNA in its 30S subunit and 23S and 58 RNAs in the 50S subunit. The functional significance of the different types of ribosomal RNA and their interactions with ribosomal proteins will now be taken up.16S rRNA is about 1,600 nucleotides long. It consists of a single strand folded upon itself to form double-stranded hairpin loops. About 30-35% of the bases occur in the single stranded loops. Twenty of the 21 ribosomal proteins have been shown to be associated with 16S rRNA and the remaining protein (82) may also be found to have binding properties with 16S rRNA
(i) The role of 16S rRNA in the function of the 30S subunit was established by Shine and Dalgarno (1974). A 7-nucleotide polypyrimidine sequence near the 3' end of 16S rRNA was found to have a complementary polypurine sequence in mRNA. This polypurine sequence preceeds the initiation codon AUG. Ribosomal binding sites of all known mRNA molecules contain a sequence of 3-7 polypurine nucleotides preceding the initiation codon. This sequence is complementary with a polypyrimidine stretch in 16S rRNA.
This interaction permits mRNA binding to the region of the 30S subunit containing the 3' end oft 68 rRNA and its associated proteins. It is in this region of the ribosome that co dons for the selection of tRNA molecules are present. Thus the 16S rRNA brings about proper orientation of mRNA and tRNA molecules in the ribosome. (ii) 238 rRNA of the 50S subunit has a rather long sequence at the 3' end that is complementary to a sequence near the Shine and Dalgarno site at the 3' end of 16S rRNA of the 30S subunit. The 23S-16S duplex thus provides a stabilizing interaction between the two subunits of the ribosome. (iii) Treatment of the 308 subunit with nuclease yields ribonuclease fragments. 16S rRNA can be divided into three specific regions (Zimmerman, 1974). A proximal fragment of about 500 nucleotides (nucleation site I) binds to proteins S4,S16,S17 and 820.
A central nucleation site IT (550-850 nucleotides) interacts with proteins S6, S8, SI5, and S18. Nucleation site III comprises the 3' one-third of 16S rRNA and binds to proteins S7, S9, SI3 and 819. (iv)168 rRNA also provides a binding site for tRNA. (v) The initiation factor IF-3 has some affinity for rRNA. It has been suggested that a segment of 16S rRNA provides a binding site for IF-3 close to its 3' end. 16S rRNA thus helps to mediate an interaction between IF-3 and mRNA.
	5S rRNA. It has been shown that 5S rRNA forms a specific complex with proteins L5, LI8 and L25 of the 50S ribosomal subunit. This 5S rRNA-protein complex binds to the G- T -φ-C sequence present in loop IV of all tRNAs. It has been suggested that this binding takes place through a complementary C-A-A-G sequence in 5S rRNA. Thus 5S rRNA has all important role in binding aminoacyl tRNA to the A site. 23S rRNA shows base complementarity between its two ends. A 7-nucleotide segment near the 3' end has been found complementary to 7 bases at the 5' end. Complementary bases are also found between 23S rRNA and 5S rRNA.

Nucleotides 72-83 of SS rRNA have been found to be Complementary to a 12-nucleotide sequence at the 3' proximal part of 23S rRNA (Herr and Noller, 1975).

From these observations it appears that the 3' and 5' ends of both 168 and 238 rRNAs are together with the initiation site of the phage. The proteins L18 plus either L5 or L25 bind 5S rRNA to 23S RNA. IF-3 is also found to be associated with the 3' end of 23S rRNA.

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Ribosomes of Chloroplasts Sedimentation Coefficients of Chloroplast and Cytoplasmic ribosomes and their Subunits in Plants RNA Content of Chloroplast Chloroplast and Cytoplasmic Ribosomes Homology Between Chloroplast and Bacterial Ribosomes Ribosomal RNA Interactions Polyribosome Biogenesis of Ribosomes Functions of Ribosomes

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