Expression of α- and β-tubulin genes during development of sea urchin embryos

Mature unfertilized eggs of the sea urchin Lytechinus pictus contain multiple α-tubulin mRNAs, which range in size from 1.75 to 4.8 kb, and two β-tubulin mRNAs, 1.8 and 2.25 kb. These mRNAs were found at similar levels throughout the early cleavage stages. RNA gel blot hybridizations showed that prominent quantitative and qualitative changes in tubulin mRNAs occurred between the early blastula and hatched blastula stages. The overall amounts of α- and β-tubulin mRNAs increased two- to fivefold between blastula and pluteus. These increases were due mainly to a rise in a 1.75-kb α RNA and a new 2.0-kb β RNA. Other, minor changes also occurred during subsequent development. All size classes of α- and β-tubulin RNAs in early and late embryos contained poly(A)⁺ translatable sequences. As reported earlier, some of each of the α RNAs, but neither of the β RNAs, are translated in the egg and a small portion of each of the stored α and β RNAs is recruited onto polysomes within 30 min of fertilization. In the work described here, subsequent development up to the morula stage was accompanied by a gradual recruitment of tubulin mRNAs into polysomes. By the early blastula stage, most of the maternal tubulin sequences were associated with polysomes. In contrast to the gradula recruitment of maternal sequences throughout cleavage, the tubulin mRNAs which appeared at the blastula stage showed no delay in entering polysomes. The exact fraction of each mRNA that was translationally active at later stages varied somewhat among the individual mRNAs. From the differential hybridization patterns of egg, embryo, and testis RNAs to various tubulin cDNA and genomic DNA probes, it is concluded that at least one gene producing maternal α mRNA is different from a second one which is expressed only in testis. Each of the three embryonic β RNAs is encoded by a different β gene; at least two of these different β genes are also expressed in testis.