Ribonucleoprotein particles derived from the lampbrush chromosomes of newt oocytes

Abstract

A particulate fraction that contains both RNA and protein has been isolated from homogenates of newt oocytes. Several lines of evidence indicate that these particles are derived from the ribonucleoprotein matrix of the oocyte chromosomes: (1) they are rapidly labelled with radioactive precursors of both RNA and protein; (2) according to measurements of density and ultraviolet absorbance ratios, the particles contain less than 3% RNA; (3) an increase in the amount of labelled RNA and protein in the particulate fraction results from treating the oocytes with a high concentration (25 μg/ml) of actinomycin D, a treatment that releases the ribonucleoprotein matrix from the chromosomes; (4) microscopic examination has shown the particles isolated from homogenates to be similar in size (up to 2 μm) and appearance to the particles released from chromosome preparations; (5) antiserum prepared against the constituent protein of the particles specifically binds with the lampbrush loops of oocyte chromosomes; (6) the base composition of RNA extracted from the particles (low G + C content) differs from that of ribosomal RNA and reflects rather the base composition of DNA. In certain circumstances, however, the particle fraction has an exceedingly high (60%) uridylic acid content. The protein of the ribonucleoprotein particles is not fortuitously bound but tends to aggregate on isolation. When solubilized in sodium dodecyl sulphate and 2-mercaptoethanol the aggregates resolve, on gel electrophoresis, to a few individual, non-basic proteins. The RNA is unique in that it differs both from messenger RNA released from polyribosomes and from ribosomal RNA, in being more rapidly labelled and in having a higher and more variable distribution of sedimentation coefficients (10 S to 50 S and more). In many respects it resembles the heterogeneous nuclear RNA described in other systems. It is suggested that the ribonucleoprotein particles represent primary transcription units of polycistronic RNA, stabilized with protein and perhaps processed by the addition of poly(U) segments, which are released into the nucleoplasm. These particles may represent stored informational units accumulated during oogenesis and activated after fertilization.