LA PROTEINS FROM XENOPUS-LAEVIS - CDNA CLONING AND DEVELOPMENTAL EXPRESSION

In mammalian nuclei, newly-synthesized RNA polymerase III transcripts are transiently associated with a phosphorylated polypeptide of ∼50 kDa called the La protein. Here we provide evidence that the frog Xenopus laevis contains mRNAs for two highly related La proteins, each apparently encoded by a single gene. Both forms of the La protein contain the RNP-80 motif previously identified in many RNA binding proteins. The steady state levels of La mRNAs and protein are approximately constant in oocytes, eggs and embryos. This implies a progressive and severe decrease in these levels on a per cell basis during early development. In particular, neither the La mRNA nor protein level increases at the mid-blastula transition, the time when RNA polymerase III transcription first occurs during embryogenesis.