Biogenesis of Ribosomes

In eukaryotes the site of synthesis of most of the ribosomal RNA (rRNA) is the nucleolus. The nucloelor organizer contains many copies of ribosomal DNA (repetitive DNA). Several distinct types of rRNA have been isolated from cells. The other types are intermediate stages in the formation of the RNA of the ribosomes. The scheme for the biosynthesis is as follows:

(1) The RNA cistron of nucleolar DNA transcribes 45S precursor RNA in the presence of the enzymes RNA polymerase. Observations on Xenopus laevis oocyptes indicate that each rDNA cistron is simultaneously acted upon by about 100 RNA polymerases, each of which transcribes one 45S RNA.

(2) The ribose sugar of certain regions of 45S RNA undergoes methylation. It is the mythylated regions which gives rise to 28S and 18S RNA of the ribosomes. The nonmethylated regions have a higher content of guanine and cytosine than the methylated regions.

(3) Cleavage at site 1 removes the transcribed spacer sequence from the 5’P end of 45S RNA.  Cleavage at site 2 separates 18S rRNA. Cleavage at site 3 results in a large segment containing 28S RNA and 5.8S RNA along with spacer segments. Cleavage 4 results in the final trimming of the segment.

(4) 5S RNA is synthesized outside the nucleolus. Treatment of cultured cells with actinoymcin D inhigits production of 28S and 18S rRNA but not 5S RNA. Also, in Xenopus laevis 5S rRNA is present in mutants without nucleoli, but not 28S and 18S RNA. These observations indicate that the genes for 5S rRNA are not located in the nucleolar organizer. The genes coding for 5S rRNA have a large number of copies in X. laevis and HeLa cells much larger than 28S and 18S genes. In Drosophila, however, the number of 5S genes and 18S and 28S genes is about the same.



(5) Ribosomal proteins are synthesized in the cytoplasm and translocated to the nucleus were they become associated with RNA. Structural core proteins first associate with RNA. Structural core proteins first associated with 45S RNA to form 80S ribonucleoproteins particles other proteins are probably bound later. The series of degradations by which 45S RNA forms 18S, 5.8S and 28S RNA occur within the ribonucleoproteins particles. Ultimately the 40S and 60S subunits of the ribosome are formed, containing 18S and 28S+5.8S RNA, respectively. 5S RNA, which is synthesized outside the nucleolus, also becomes associated with the large 60S subunit. The two types of subunits pass through pores in the nuclear membranes to the cytoplasm.

(6) The 40S subunit binds to mRNA in the cytoplasm to form a 40S-mRNA complex. The 60S subunit now becomes associated with the 40Sm-RNA complex to form the 80S ribosome with bound mRNA.

(7) In prokaryote cells the subunit RNA precursors are trimmed to from 16S and 23S RNA. These become associated with protein to form the 30S and 50S subunits, respectively, of the 70S ribosome. The RNA genes are present in only a few copies closely grouped together, i.e., the ribosomal genes occur in a single operon.