The determination of the molecular weight of ribonucleic acid by polyacrylamide-gel electrophresis. The effects of changes in conformation.

1. The effects of changes in experimental conditions on the mobility of RNA in polyacrylamide-gel electrophoresis were investigated. 2. The linear relation between log(molecular weight) and electrophoretic mobility was shown to be independent within limits of salt or gel concentration. 3. The relative mobility of RNA with low content of guanylic acid and cytidylic acid residues was decreased in low-ionic-strength buffer. This was related to a small relative decrease in sedimentation coefficient. 4. However, Mg(2+) ion caused almost no increase in mobility although it was associated with large increases in sedimentation coefficient. This suggested opposing actions of Mg(2+) ion on the size and effective charge of the RNA. 5. It is concluded that the method provides a satisfactory measurement of molecular weight, which is almost independent of the nucleotide composition of RNA at moderate salt concentrations.