Poly(A)(+) RNA during vegetative development of Acetabularia peniculus

In the juvenile stage, the diploid giant-celled green algae Acetabularia sp p. are differentiated into an upright stalk and an irregularly branched rhi zoid. Early amputation and grafting experiments as well as biochemical and molecular analyses have shown that mRNA (as poly(A)(+) RNA) is continuously supplied from the primary nucleus in the rhizoid and accumulates in the st alk apex. In the present study, localization of poly(A)(+) RNA in the juven ile stage of the Acetabularia peniculus was investigated by fluorescent in situ hybridization using oligo(dT) as a probe. The signal was localized in the apical cytoplasm and, in addition, multiple longitudinal striations thr oughout the stalk and rhizoid cytoplasm. A large portion of the poly(A)(+) RNA striations exhibited structural polarity, broadened at one end and grad ually thinned toward the other end. Some of the striations in the rhizoid c ytoplasm were continuous with a zone of signal in the area of the perinucle ar rim. The poly(A)+ RNA striations were associated with thick bands of lon gitudinal actin bundles which run through the entire length of the stalk. C ytochalasin D caused fragmentation of the actin bundles and irregular distr ibution of the fluorescent signal. We suggest that the poly(A)(+) RNA stria tions constitute a hitherto unknown form of packaged mRNA that is transport ed over large distances along the actin cytoskeleton to be stored and expre ssed in the growing apex.