Electrophoretic characterization of bacterial polyribosomes in agarose-acrylamide composite gels

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Bacterial polyribosomes, ribosomes and ribosomal subunits were found to possess characteristic electrophoretic mobilities in polyacrylamide-agarose composite gels (containing 2 to 3% acrylamide). The mobilities of these particles were inversely proportional to their size. The smaller (30 s) subunit had the fastest mobility, and migrated at a rate similar to its constituent 16 s RNA. Polyribosomes containing eight ribosomes were the largest that could be resolved. Brief treatment with RNase converted the polyribosomes to faster migrating particles (70 s ribosomes). A unique feature of the electrophoretic analysis was the ability to identify, by a second electrophoresis, the type of RNA contained in ribosomal subunits, ribosomes and polyribosomes. Similarly, the subunit composition of ribosomes and polyribosomes could be established. The principal subunits observed were a faster (30 s) subunit containing 16 s RNA and a slower (50 s) subunit containing 23 s RNA. Polyribosomes resembled each other in their proportional content of these subunits. RNA isolated from polyribosome preparations was composed of three very similar but electrophoretically distinct 23 s RNA's; similarly the 16 s RNA was composed of two distinct RNA's. Another RNA migrating somewhat slower than the 16 s RNA (16 s–17 s) was observed. In addition to these multiple forms of RNA an unanticipated diversity of subunits was observed. One of the smaller subunits contained the slower 16 s–17 s RNA and may represent the 26 s precursor particle of the 30 s subunit. It is likely that some of the variations in subunit mobilities are related to differences in the size of the RNA which they contain. However, provided the protein is not removed, the electrophoretic mobilities of subunits and polyribosomes are not dependent on their containing intact RNA.