A study of the sedimentation properties of the U-snRNPs of mouse embryoid bodies (OTT6050) by sucrose gradient centrifugation under physiological ionic conditions and electrophoretic analysis of the RNA in various fractions.

Fractionation of nuclear extracts of mouse embryoid bodies (EBs; OTT6050) on sucrose gradients (5-20%) under conditions of physiological ionic strength (150 mM NaCl, 1mM MgCl2) and an analysis of the RNA in various fractions by gel electrophoresis revealed that the major U small nuclear ribonucleoproteins (U-snRNPs; U1a, U1b, U2, U4, U5 and U6-snRNPs) sediment over a wide region of the gradient, although they sediment preponderantly in the light region of the gradient. This result suggests that, under these experimental conditions, some of the populations of snRNPs exist as free particles separated from large nuclear RNP particles, while some of the populations associate with them in EBs. Furthermore, all species of these major U-snRNPs appear to associate with the larger nuclear RNP particles of EBs, since all these species sediment in the heavier fractions (approximate greater than or equal to 60S) of the 15-40%/50% gradients. The relative abundance of the various species of major U-snRNPs can also be observed to vary among the fractions of the gradients. A similar analysis of the post-mitochondrial cytoplasmic fraction showed that some leakage of the major U-snRNPs, but not the selective leakage of any particular species of U-snRNP, from the nuclear fraction, occurred during aqueous fractionation of the cells. Some species of RNA, larger than the snRNAs U1a/b and U2 respectively were also detected in the cytoplasmic fraction.