In vivo storage of XR family interspersed RNA in Xenopus oocytes.

Interspersed RNA is an abundant class of cytoplasmic poly(A)+ RNA which contains repetitive elements within mostly heterogeneous single copy sequences. In spite of its quantitative importance in oocytes or eggs (two-thirds of the total poly(A)+ RNA), very little is known about its synthesis, its interaction with other molecules, and its functional significance. Here, we analysed a prevalent family of interspersed RNA (XR family) during Xenopus oogenesis. We found that XR interspersed RNA, unlike extracted interspersed RNA, did not form RNA duplexes in vivo. In small oocytes (stage III), XR RNA interacted with proteins forming rapidly sedimenting ribonucleoprotein particles (RNPs) with a median sedimentation constant of 80S. However, towards the end of oogenesis (stage VI), these XR RNPs changed into smaller particles with a median sedimentation constant of 40S. By analysing the proteins associated with XR RNA sequence, we have identified a 42 kilodalton protein in small oocytes, which was replaced by a 45 kilodalton protein at stage V of oogenesis.