A subcloned portion of the 5' nontranslated sequence from a Physarum alpha-tubulin cDNA is specific for a single alpha-tubulin locus, altB, of Physarum polycephalum. We find that this locus is expressed only in the plasmodium and encodes at least an alpha 1-tubulin isotype, which we have designated alpha 1B. Hybridization patterns of other subclones of this cDNA reveal two sequences for alpha-tubulin at the altB locus.

Physarum myxamoebae can be reversibly induced to become flagellates. Physarum flagellates contain a new form of tubulin, alpha 3, that is not found in nonflagellated cells. Evidence is presented that suggests that alpha 3 tubulin arises through posttranslational modification of a preexisting alpha tubulin. Pulse-chase experiments showed that labeled alpha 3 tubulin could be detected when flagellates formed after a chase. RNA was isolated from myxamoebae at different times after induction of flagellum formation. When this RNA was translated in vitro, the resulting products contained no alpha 3 tubulin, also consistent with alpha 3 being made by posttranslational modification. Levels of alpha and beta tubulin RNA increased with the proportion of flagellates in the culture. These elevated tubulin RNA levels declined after the number of flagellates in the population achieved plateau values.