unit of measurement used to describe the ribosomal subunits and the rRNA fragments is the Svedberg unit, a measure of the rate of sedimentation in centrifugation rather than size. This accounts for why fragment names do not add up: for example, bacterial 70S ribosomes are made of 50S and 30S subunits. Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. E. coli, for example, has a 16S RNA subunit (consisting of 1540 nucleotides) that is bound to 21 proteins. The large subunit is composed of a 5S RNA subunit (120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 31 proteins. {| class="wikitable float-right" style="text-align:center" |+ Ribosome of E. coli (a bacterium) |- ! width="25%"| ribosome ! width="25%"| subunit ! width="25%"| rRNAs ! width="25%"| r-proteins |- | rowspan="3" | 70S || rowspan="2" | 50S || 23S (2904 nt) || rowspan="2" | 31 |- | 5S (120 nt) |- | 30S || 16S (1542 nt) || 21 |} Affinity label for the tRNA binding sites on the E. coli ribosome allowed the identification of A and P site proteins most likely associated with the peptidyltransferase activity; labelled proteins are L27, L14, L15, L16, L2; at least L27 is located at the donor site, as shown by E. Collatz and A.P. Czernilofsky. Additional research has demonstrated that the S1 and S21 proteins, in association with the 3′-end of 16S ribosomal RNA, are involved in the initiation of translation. Archaeal ribosomes Archaeal ribosomes share the same general dimensions of bacteria ones, being a 70S ribosome made up from a 50S large subunit, a 30S small subunit, and containing three rRNA chains. However, on the sequence level, they are much closer to eukaryotic ones than to bacterial ones. Every extra ribosomal protein archaea have compared to bacteria has a eukaryotic counterpart, while no such relation applies between archaea and bacteria. Eukaryotic ribosomes Eukaryotes have 80S ribosomes located in their cytosol, each consisting of a small (40S) and large (60S) subunit. Their 40S subunit has an 18S RNA (1900 nucleotides) and 33 proteins. The large subunit is composed of a 5S RNA (120 nucleotides), 28S RNA (4700 nucleotides), a 5.8S RNA (160 nucleotides) subunits and 49 proteins. {| class="wikitable float-right" style="text-align:center" |+ eukaryotic cytosolic ribosomes (R. norvegicus) |- ! width="25%"| ribosome ! width="25%"| subunit ! width="25%"| rRNAs ! width="25%"| r-proteins |- | rowspan="4" | 80S || rowspan="3" | 60S ||