cDNA/RNA hybridization experiments of polysomal and nuclear poly(A)-rich RNA from early tadpole stages of ,Yenopus luevis revealed that part of the nuclear poly(A)-rich RNA sequences are not present within the polysomal polyadenylated RNA. For a more detailed analysis of these sequences we have cloned double-stranded cDNA derived from tadpole nuclear poly(A)-rich RNA in the PstI cleavage site of pBR 322. By colony screening with 32P-labelled cDNA from polysomal and nuclear poly(A)-rich RNA of the tadpole stage we could identify and isolate some of the cloned sequences, which are present only within the nuclear RNA. However, hybridization with cDNA from polysomal poly(A)-rich RNA of the gastrula stage indicated that at least one of those sequences which are confined to the nucleus at tadpole stage may serve as mRNA at gastrula stage. We present evidence tliat nuclear and polysomal poly(A)-rich RNA molecules containing the same nucleotide sequence differ in size and that size reduction at the level of processing precedes and may enable cytoplasmic export. We conclude that, besides stage-specific regulation of transcription, post-transcriptional control mechanisms are also involved in gene expression during embryonic development.The application of recombinant DNA technology has produced a very effective approach to the elucidation of differential gene expression during early embryonic development. Dramatic changes in the abundance of individual mRNA species during development have been well established for many organisms [I -111. The amount of certain mRNA species may be regulated at the level of transcription but posttranscriptional control mechanism can also account for such changes.In the present investigation we have looked for the existence of post-transcriptional control mechanisms during the development of the South African clawed toad, Xenopus luevis, by analyzing the polyadenylated nuclear and polysomal RNA species. The physiological significance of the addition of a poly(A) tail to eucaryotic mRNA, ten years after its discovery [12-161, still remains an open question. For the majority of mRNA sequences coding for structural proteins such as globin, addition of poly(A) represents an obligatory step in the processing of the primary transcripts. On the other hand it is generally accepted today that in most tissues part of tlie mRNA population lack poly(A) stretches. These sequences may encode for the same proteins as the poly(A)-rich mRNA fraction, but some sequences may encode for different proteins (for review see [17]). From these results it may be concluded that the poly(A) addition does not constitute an essential general mechanism in the expression of protein-coding genes.Another intriguing question is whether the polyadenylaLion step implies mandatory transport of these molecules into the cytoplasm. It has been shown for many organisms and tissues that base sequence complexity of nuclear polyadenylated RNA is much higher than that of cytoplasmic or polysoma1 poly(A)-rich RNA [18-231. This suggests th...
