PROGRAMMED DNA-DEGRADATION AND NUCLEOLAR BIOGENESIS OCCUR IN DISTINCTORGANELLES DURING MACRONUCLEAR DEVELOPMENT IN TETRAHYMENA

Programmed DNA rearrangements, including DNA degradation, characterize the development of the soma from the germline in a number of developm ental systems. Pdd1p (programmed DNA degradation 1 protein), a develop ment-specific polypeptide in Tetrahymena, is enriched in developing ma cronuclei (anlagen) and has been implicated in DNA elimination and nuc leolar biogenesis Here, immunocytochemistry and fluorescent in situ hy bridization (FISH) were employed to follow Pdd1p and two nucleolar mar kers (Nopp52 and rDNA) during macronuclear development. Both Pdd1p and Nopp52 localize to subnuclear structures, each of which resemble nucl eoli. However, while true nucleoli form and persist during development , Pdd1p-positive structures are only present for a brief period of mac ronuclear differentiation. Accordingly, two distinct organelles can be recognized in anlagen: (1) Pdd1p-positive structures, which lack Nopp 52 and rDNA, and (2) developing nucleoli which contain rDNA and Nopp52 but lack Pdd1p. Taken together with recent data corroborating Pdd1p's role in DNA elimination, we favor the hypothesis that Pdd1p structure s are unique, short-lived organelles, likely to function in programmed DNA degradation and not in nucleolar biogenesis.