| Abstract: | A study of the action of ribonuclease on 30--50-S monoparticles prepared from pre-messenger ribonucleoprotein (pre-mRNA . protein) was started in order to elucidate the structure of monoparticles. A ribonucleoprotein complex containing mostly 30000--38000-Mr proteins of pI 7--9 (alpha class) persisted under conditions where other proteins (23000--110000 Mr, pI 5--8.5, beta class) were relased. An unexpected increase of sedimentation coefficient accompanied the formation of the ribonucleoprotein complex. The extent of increase varied with the initial size of the monoparticles, reaching 45% for 30-S monoparticles. The ribonucleoprotein complexes designated here as 40--45-S alpha-ribonucleoproteins were more homogeneous in size than the original monoparticles. Electron microscopic examination showed that the sedimentation shift corresponded to an increase of the actual size of the particles, not to flattening or change of shape. Therefore, the 40--45-S alpha-ribonucleoprotein is not a pre-existing unit of pre-mRNA . protein but arises from specific rearrangements probably between small alpha ribonucleoproteins formed by fragmentation of monoparticles. In addition to the 40--45-S alpha-ribonucleoproteins, large protein aggregates corresponding to 15% of the monoparticle proteins were formed upon ribonuclease hydrolysis. Their major proteins were neutral, suggesting that the aggregates might be precipitates of proteins at pH close to the pI. Ribonuclease being a widespread cellular enzyme, partial rearrangements may occur during preparation and handling of pre-mRNA . protein. It is particularly crucial to remark that the 40--45-S alpha-ribonucleoprotein which does not pre-exist might be mistaken for a pre-mRNA . protein unit. |
