Nuclear RNA in sea urchin embryos : I. Some characteristics of ribonucleoprotein complexes

Using cesium chloride gradient analysis, we have examined the kinetics of labeling and some physical properties of nuclear ribonucleoprotein complexes in sea urchins. Our results strongly indicate that these complexes are not artifacts of procedure but are definite members of chromatin that are readily distinguishable from cytoplasmic RNP complexes. The nuclear complexes can be separated into two major classes: (1) A class in which the protein to RNA ratio is at least 4:1; (2) a second class in which the protein to RNA ratio is much less. Using ³H]uridine, long-term labeling and short pulses of ³²P], we have studied the labeling kinetics of RNA in the two classes. The nuclear RNA which is associated to a high degree with protein turns over very rapidly; over 70% of the nuclear RNA labeled in a short pulse appears in this fraction, whereas the nuclear RNA synthesized in long-term pulses is present to a greater extent in complexes containing a much smaller proportion of protein. The difference in the rate of turnover of RNA in these two classes may be significant in the regulation of RNA processing in the nucleus.