RESUMO
In this study we show that the onset of embryonic transcription in the marine snail Patella vulgata coincides with the start of the sixth cleavage, when the cell-cycle elongates and divisions become asynchronous. Changes in mRNA content before and after onset of transcription were initially demonstrated by in vitro translation of isolated mRNA from different stages. Before the sixth cleavage, three major mRNAs encoding proteins of 36, 50 and 52 kDa were present. These proteins probably correspond to cyclin A and B and ribonucleotide reductase. After this stage, three major proteins with molecular weights of 36.5, 52.5 and 53 kDa were found after in vitro translation. Via hybrid selected translation and differential screening cDNAs corresponding to the 52.5 and 53 kDa proteins were cloned. The encoded proteins resemble tubulins from other animals to a high extent (between 96.5 and 93.1% identity for α-tubulin and 97.9 and 75.9% for ß-tubulin). The 36.5 kDa protein is the previously described actin. Both tubulins were expressed at or shortly after the first asynchronous division after the fifth cleavage.
