Ribosomal RNA (rRNA)

Ribosomal RNA is found in the ribosomes. In prokaryotes the rRNA molecules is formed on a part of the DNA strand called the ribosomal DNA. Ribosomes are formed on the nucleolus. The nucleolar organize contains ribosomal DNA which transcribes 45S RNA. This then gives rise to 28S RNA through a series of intermediate steps. Ribosomal RNA is formed from only a small section of the DNA molecule, and hence there is no definite base relationship between rRNA and DNA as a whole.

Ribosome RNA consists of a single strand twisted upon itself in some regions. It has helical regions connected by intervening single strand regions. The helical regions may show presence or absence of positive interaction. In the helical region most of the base pairs are complementary, and are joined by hydrogen bonds. In the unfolded single strand regions the bases have no complements. Hence rRNA does not show purine-pyrimidine equality. The rRNA strands upfold upon heating and refold upon cooling. Ribosomal RNA is stable for atleast two generations.

The ribosome consists of proteins and RNA. The 70S ribosome of Prokaryotes consists of a 30S subunit and a 50S subunit. The 30S subunit contains 16S rRNA, while the 50S subunit contains 23S and 5S rRNA. The 80S eukaryote ribosome consists of a 40S and a 60S sudunit. In vertebrates the 40S subunit contains 18S rRNA while the 60S subunit contains 28-29S, 5.8S and 5S rRNA.

Synthesis of rRNA

In bacterial genes the sequences specifying 16S, 23S and sometimes also 5S RNA are arranged in a series. mRNA is transcribed from DNA as a 30S unit, which has been called P30S. During processing the P30S transcriptional unit is cleaved within the spacer segment by RNase III 25S and 18S segments. These are then reduced to p23S and p165 segments respectively, also by RNase III. Secondary trimming of these intermediates yields the final size, 23S and 16S, respectively. The enzyme involved in secondary trimming is probably a ribonuclease, designated as RNase M or maturase.

The 5’ and 3’ terminal pieces are broken down to their monouculeotides by an exonuclease, which thus serves a scavenging function. Mature 5S rRNA is resistant to this enzyme. The precursor P5B is also split by RNase M5 into a 22 nucleotide 5’ terminal piece, a mature 5S mRNA and some smaller 3’ terminal pieces.