Visualisation in the Electron Microscope and Size of RNA from Animal Cells

Ribosomal, preribosomal (pre-rRNA) and messenger like (mlRNA) RNA molecules from animal cells have been spread in extended form for visualisation in the electron microscope, in order to measure their length. It is shown that the giant nascent pre-rRNA and mlRNA consists of linear molecules of homogeneous shape. Their length ranges up to 3.5 μ in the case of pre-rRNA and up to 8.5 μ for mlRNA. The correlation of apparent lengths and molecular weight of RNA is discussed. A spacing of 2.45 A, based on the length and the chemically determined molecular weight of E. coli rRNA, allows one to assign molecular weights to rRNA and pre-rRNA that are in fairly good agreement with published data based on sedimentation analysis. However, the 28 S rRNA has twice the length of the 18 S species, which is in disagreement with the mass relation of 2.5–3.0 observed by sedimentation. In the case of mlRNA the spacing of 3.17 A, derived from the length to molecular weight relation of R 17 phage RNA, is more likely to give correct values than that based on rRNA. This conclusion derives from a discussion of the relation of length and molecular weights in various viral RNA molecules in a range of up to 107 daltons. Two general conclusions may be drawn from the work presented: 1 The high sedimentation rate of pre-rRNA and mlRNA is not based on artificial agglomeration or particular configuration but on the real mass of molecules which contain, in the case of mlRNA, up to 30,000 bases (molecules of up to 50,000 bases are likely to exist also) in covalent linkage. 2 The electron microscope may be used to determine molecular weights. The precision of the values obtained may be equal to or better than those determined by sedimentation if correct spacing data are available from reference molecules of the same RNA class.