Ribosomal RNA

The immature rrn transcript can form stable secondary structures made up of stems of RNA: RNA complementary duplexes and loops of single-stranded RNA. The structure allows the protein to bind to the rRNA initially to form a ribonucleoprotein (RNP) complex. Some of these proteins remain to become part of the mature ribosome but some are lost as 24 base-specific methylations of the rRNA take place.

It is also at this stage which is the first RNase III stage of maturation occurs. Further processing of the precursors through M5, M16 and M23 RNases generates three ribosomal RNA species named 16S, 23S and 5S. The ‘S’ stands for Svedberg units and refers to the sedimentation coefficient – an older measure of molecular weight and size. With the advent of sequencing the sizes of the rrn genes are equally well known 16S being 1541 bp 23S 2904 bp and the smallest 5S is only 119 bp in E. coli.