Preferential expression of one -tubulin gene during flagellate development in Physarum

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Abstract
The microbial eukaryote Physarum polycephalum displays several distinct cell types in its life cycle, including amoebae, flagellates and plasmodia. Despite its relative simplicity, Physarum has a tubulin gene family of complexity comparable to that of Drosophila. We have identified beta-tubulin cDNAs from Physarum that are derived from the betA beta-tubulin locus and encode beta 1A tubulin. We have also identified a partial cDNA for the unlinked betB beta-tubulin gene, which encodes beta 1B tubulin. The polypeptide sequences encoded by betA and betB show 99% identity, but the nucleotide sequences show only 85% identity, consistent with an ancient duplication of these genes. The betB gene is expressed in amoebae, flagellates and plasmodia, whereas betA is expressed only in amoebae and flagellates. During the amoeba-flagellate transition the level of betA transcript increases over 100-fold, while the level of betB transcript changes very little. Thus Physarum has a mechanism for regulating the level of discrete beta-tubulin transcripts differentially during flagellate development. A need for this differential regulation could account for the maintenance of the virtually isocoding betA and betB beta-tubulin genes.

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