The Control of Maternal Messenger RNA Expression During the Early Development of <em>Xenopus laevis</em>: A Thesis

Abstract

Maternally inherited poly(A)+ RNAs are important for directing early development in many animal species. This thesis investigates the regulation of maternal mRNA in the South African clawed frog, Xenopus laevis. The first portion of this thesis examines an unusual class of maternal RNA, interspersed poly(A)+ RNA, which is composed of co-linear repeat and single copy sequences. A cDNA clone, called pXR, contains the repeat portion of an interspersed RNA that hybridizes to several different oocyte transcripts of diverse size that persist until the neurula stage. DNA sequence analysis of the cDNA and hybrid selection of the oocyte transcripts followed by in vitro translation show that molecules of this repeat family are not translatable. This data, combined with the developmental profile of XR containing RNAs, indicate that members of this repeat family are not likely to be maternal messenger RNAs. The second part of this thesis investigates the expression of a class of maternal mRNAs that are regulated by cytoplasmic polyadenylation during progesterone induced oocyte maturation. One particular mRNA G10, is stored as a polyadenylated RNA in the cytoplasm of stage VI oocytes until maturation when the process of poly(A) elongation stimulates its translation. Injection of mutant and wild-type mRNAs, synthesized in vitro, revealed that two sequence elements, UUUUUUAUAAAG and AAUAAA, were both necessary and sufficient for polyadenylation and polysomal recruitment of G10. Maturation promoting factor and cyclin as well as progesterone can induce polyadenylation but in each case protein synthesis is required. Extracts from oocytes and unfertilized eggs were employed to identify factors that may be responsible for maturation-specific polyadenylation. An 82 kd protein that binds to the UUUUUUAUAAAG in egg, but not oocyte extracts, was identified by UV crosslinking. This data suggests that p82 is a good candidate for a developmentally regulated protein that controls the expression of maternal messenger RNAs in early Xenopus development.