HEADLINES

Short Inverted repeats at a free end signal large palindromic DNA formation in Tetrahymena

L. F. Yasuda and M-C. Yao Cell 67, 505-516

The formation of large inverted DNA duplications occurs in a variety of sys- tems where genes are expressed at high levels. These palindromes are often amplified to many copies per genome. The ciliate Tetrahymena provides an attractive model system in which to study the mechanisms underlying these events, since palindrome formation and amplification occur at a specific stage in the development of every cell. During development of a new macronucleus, the ribosomal RNA genes (rDNA), which are integrated as a single copy in the micronucleus, are excised and converted to a linear molecule containing two head-to-head rDNA copies. Telomeres are added at each end, and the extrachromosomal palindrome is amplified to about 9000 copies per macronucleus.

The authors of this study addressed the mechanism of palindrome for- mation by transforming developing Tetrahymena with particular rDNA con- structs. The results show that a pair of short inverted repeats separated by a spacer, and a double-stranded chromosome break nearby, are sufficient for palindrome formation. The symmetry, rather than the specific sequence, of the inverted repeats is important in this process. It was inferred that the chromosome breakage developmentally regulates rDNA palindrome formation in Tetrohymena.

The formation of stem-loop structures by a short inverted repeat next to a chromosome break could be a general starting point for palindrome forma- tion. Remnants of such simple features can be found by analysing amplified inverted repeats in mammalian cells and it seems reasonable to argue that they reflect a conserved mechanism of palindrome formation operating in diverse eukaryotic cells.

Phosphorylatlon at Thr167 is required for Schizosaccharomyces pomhe p34cdcz function

K. L. Gould, S. Moreno, D. J. Owen, S. Sazer and P. Nurse EMBO J. 10, 3297-3309

cdc2 phosphorylatlon Is required for Its Interaction with cyclin

B. Ducommun, P. Brambilla, M-A. Felix, B. R. Franza, Jr, E. Karsenti and G. Draetta

EMBOJ. 10, 3311-3319

Our understanding of the eukaryotic cell cycle has been greatly advanced by the knowledge that the activity of the key regulator, p34 cdc2 protein kinase, is regulated by phosphorylation and cyclin association. Here, characterization of the cell cycle-regulated phosphorylation sites on p34 cdc2 in two divergent eukaryotic systems provides evidence that threonine phosphorylation and dephosphorylation are involved in the regulation of p34cdc2 function.

In the fission yeast S. pombe, Gould et al. extended their earlier observation that dephosphorylation of Tyrl 5 activates p34cdc2 and regulates the initiation of mitosis, by identifying a second phosphorylation site in the protein, Thr167. Ducommun et ol. found that the equivalent residue (Thr161) in human p34cdc2 is critical to p34cdc2 kinase function. Together, the results show that phosphorylation of this threonine residue is essential for kinase activity and the association of p34 cdc2 with cyclins A and B. The replacement of the Thr residue with Glu or Asp by site-directed mutagenesi~ i. ~ an attempt to mimic constitutive phosphorylation resulted in misregulated mitotic entry and cytokinesis. The identity of the proteins regulating phosphorylation and dephosphorylation at this site, and their regulation through the cell cycle, will be interesting topics for future research.

Tubular early endosomal networks in AtT20 and

other cells

J. Tooze and M. Hollinshead J. Cell BioL 115, 635-653

The morphology of endosomal com- partments in cells has been the sub- ject of much discussion recently. The possible existence of an endosomal reticulum and whether that includes both early and late endosomes is one of the focuses of the debate. The amount of communication within and between endosomal compart- ments is important for our under- standing of sorting and recycling in the endocytic pathway.

The authors of this paper demon- strated the existence of tubular endosomes in many different cell lines by electron microscopy of rela- tively thick sections (0.2-0.5 I~m) of cells loaded with horseradish peroxi- dase. The tubular endosomes were found both at the cell periphery and in the Golgi complex region around the centrioles. The authors showed that the tubular endosomes were likely to be early endosomes that form small branching networks and that these were distinct from late endosomes. The morphology of the tubular structures did not depend on the presence of intact micro- tubules.

Further data showing the exclusion of late endosome markers such as the mannose 6-phosphate receptor from the tubular endosomes will be required to confirm the designation of tubular endosomes as early endo- somes. Together with other recent work suggesting the existence of an endosomal reticulum, these findings suggest that our view of communi- cation in the endocytic pathway will need to be adapted to accommodate an interconnecting system contain- ing endocytosed material, at least at early times after internalization.

HEADLINES The HEADLINES section of Trends in Cell Biology is intended to provide short summaries of the data and implications of a selection of recent papers of interest to cell biologists. HEAD- LINES are contributed regularly by a panel of research scientists appointed by the Editor.

40 TRENDS IN CELL BIOLOGY VOL. 2 FEBRUARY 1992