Abstract
1. Electrophoresis on cellulose acetate membrane in a tris–pyrophosphate buffer was used to separate microsomal fractions into three components: (1) the lipoprotein; (2) the nucleoprotein (termed the β-band); (3) traces of free RNA (termed the α-band). In tris buffer containing Mg2+ the α-band was not obtained. 2. The incorporation of uridine and phosphate into RNA by excised pea-seedling root segments was studied by using this electrophoretic technique. 3. It was shown that after a short (`pulse') incubation in the radioactive precursor and a longer (`chase') incubation in the non-radioactive precursor most of the incorporation was into the RNA of the α-band and little into that of the β-band. Previous work showed that in roots of whole seedlings the incorporation is mostly into the ribosomal RNA, corresponding to the material in the β-band. 4. A pulse-labelled RNA has also been found; this seems to be a cell fraction distinct from the microsomes or ribosomes. 5. The apparent base compositions of labelled RNA in the α-band and small amounts of labelled RNA in the β-band and of unfractionated RNA were very different from the composition of ribosomal or transfer RNA, and somewhat like that of DNA. 6. It is suggested that the excised root segment synthesizes a messenger-RNA fraction labelled after a pulse incubation and a distinct messenger RNA labelled after a pulse and chase incubation, but no ribosomal or transfer RNA. The system is thus similar to the `step-down' culture conditions in bacteria.
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Selected References
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