Photomorphogenesis in Physarum polycephalum. Temporal expression pattern of actin, alpha- and beta-tubulin.

Photo-induced fruiting-body formation of the slime mold Physarum polycephalum can be divided into two stages. The first stage (I) starts with the beginning of illumination (0 h) and ends with the formation of nodules (12 h). The second stage (II) is characterized by culmination, sporangiophore formation and melanization of sporangiophore heads (13-17 h). We investigated the expression of actin, alpha- and beta-tubulin during this differentiation process using Northern blot analysis and run-off transcription in isolated nuclei. Whereas actin mRNA is irreversibly lost during stage I, we observed two peaks in mRNA concentration for alpha-tubulin and one peak for that of beta-tubulin during stage I and a coordinated alpha/beta-tubulin mRNA induction during stage II. Stage II induction appears to be related to a presporangial mitosis. Transcriptional activity of the three genes studied shows two maxima, namely one in the middle of stage I and the other at the end of stage II. Our data suggest that the expression of the three cytoskeletal proteins investigated follows a distinct temporal pattern comprising changes in both mRNA synthesis and decay. We also propose a novel function of tubulin in Physarum which is not related to mitosis.