Can a reverse transcriptase by involved in centriole duplication?

The Cilioprotist Cytoskeleton , a Model for Understanding How Cell Architecture and Pattern Are Specified: Recent Discoveries from Ciliates and Comparable Model Systems

The cytoskeletons of eukaryotic, cilioprotist microorganisms are complex, highly patterned, and diverse, reflecting the varied and elaborate swimming, feeding, reproductive, and sensory behaviors of the multitude of cilioprotist species that inhabit the aquatic environment. In the past 10-20 years, many new discoveries and technologies have helped to advance our understanding of how cytoskeletal organelles are assembled in many different eukaryotic model systems, in relation to the construction and modification of overall cellular architecture and function. Microtubule organizing centers, particularly basal bodies and centrioles, have continued to reveal their central roles in architectural engineering of the eukaryotic cell, including in the cilioprotists. This review calls attention to (1) published resources that illuminate what is known of the cilioprotist cytoskeleton; (2) recent studies on cilioprotists and other model organisms that raise specific questions regarding whether basal body- and centriole-associated nucleic acids, both DNA and RNA, should continue to be considered when seeking to employ cilioprotists as model systems for cytoskeletal research; and (3) new, mainly imaging, technologies that have already proven useful for, but also promise to enhance, future cytoskeletal research on cilioprotists.

Centrosome-associated RNA in surf clam oocytes

Centrosomes are the major microtubule-organizing center in animal cells. They are composed of a pair of [9(3) + 0] centrioles surrounded by a relatively ill-defined pericentriolar matrix, provide the ciliary centriole-kinetosome (basal body) progenitor, and organize the assembly of microtubules into the mitotic spindle during cell division. Despite >100 years of microscopic observation and their obvious significance, our understanding of centrosome composition, dynamic organization, and mechanism of action is limited when compared with that of other cellular organelles. Centrosomes duplicate only once per cell cycle to ensure development of a normal bipolar spindle. The initial event in centrosome duplication is centriole replication, which is generative, semiconservative, and independent of the nucleus. Such observations led to the proposal that centrosomes contain their own complement of nucleic acids, possibly representative of an organellar genome comparable with those described for mitochondria and chloroplasts. The consensus in the field is that centrosomes lack DNA but may contain RNA. We isolated centrosomes from oocytes of the surf clam, Spisula solidissima, and purified from them a unique set of RNAs. We show here by biochemical means and subcellular in situ hybridization that the first transcript we analyzed is intimately associated with centrosomes. Sequence analysis reveals that this centrosome-associated RNA encodes a conserved RNA-directed polymerase domain. The hypothesis that centrosomes contain an intrinsic complement of specific RNAs suggests new opportunities to address the century-old problem of centrosome function, heredity, and evolution.

Early evolution of microtubules and undulipodia

A critique of both autogeneous and symbiotic hypotheses for the origin of microtubules and cilia and eukaryotic flagella (undulipodia) is presented. It is proposed that spirochetes provided the ancient eukaryotic cell with microtubules twice; cytoplasmic microtubules originated from phagocytosed spirochetes whereas axopodial tubules of undulipodia were transformed from ectosymbiotic spirochetes. A role in transport for microtubules in spirochetes together with a detailed scenario by which free-living spirochetes attached as ectosymbionts and subsequently differentiated into undulipodia is outlined. A mechanism for the continuity of motility in the form of "training" of the novel microtubular axoneme by the ancient spirochete motility apparatus is proposed. Transitional states (missing links) are unlikely to have survived. Constraints regarding the nature of the host cell are discussed. A corresponding flowchart of the early evolution of eukaryotes is presented in which plastids and mitochondria are polyphyletic in their origins.

Do centrioles contain ribosomes?

Centrioles induced via a number of parthenogenetic agents regularly reveal the presence of one or more granules within their central cores. Though not a new discovery, these centriolar granules have carefully been re-evaluated here. Considering various criteria, it is proposed that these granules are intra-centriolar ribosomes. More specifically, they are more comparable to 70S ribosomes than to 80S cytoribosomes. The data suggest that within the lumen of centrioles, certain centriolar proteins are synthesized on ribosomes that may be uniquely centriolar.