ONSET OF TRANSCRIPTION IN PATELLA-VULGATA COINCIDES WITH CELL-CYCLE ELONGATION AND EXPRESSION OF TUBULIN GENES

In this study we show that the onset of embryonic transcription in the marine snail Patella vulgata coincides with the start of the sixth cl eavage, when the cell-cycle elongates and divisions become asynchronou s. Changes in mRNA content before and after onset of transcription wer e initially demonstrated by in vitro translation of isolated mRNA from different stages. Before the sixth cleavage, three major mRNAs encodi ng 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 tran slation and differential screening cDNAs corresponding to the 52.5 and 53 kDa proteins were cloned. The encoded proteins resemble tubulins f rom other animals to a high extent (between 96.5 and 93.1% identity fo r alpha-tubulin and 97.9 and 75.9% for beta-tubulin). The 36.5 kDa pro tein is the previously described actin. Both tubulins were expressed a t or shortly after the first asynchronous division after the fifth cle avage.