Intracellular free calcium and mitosis in mammalian cells: anaphase onset is

calcium modulated but is not triggered by a brief transient

R.M. TOMBES AND G.G. BORISY

J. Cell Biol. 109, 627-636

The critical metaphase-anaphase transition, leading to sister chromatid separation and microtubule depolym- erization, has been thought to be trig- gered by a transient rise in intracellu- lar free calcium (Cai), although not all results are consistent with this the- ory; for example, longer term, less

dramatic increases in Ca i have also been seen. Moreover, most experi- ments have not directly measured Ca i. Tombes and Borisy reassessed the role of Ca i in the metaphase-anaphase transition by measuring Ca i with the aid of the fluorescent dye fura-2. Although Swiss 3T3 cells undergo rapid tran- sient Ca i changes, they and most fibroblasts and epithelial cells also undergo a longer term gradual increase in Cai; furthermore, remov- ing serum or calcium from the medi- um eliminated the transient changes

but did not affect the gradual rise or inhibit the metaphase-anaphase tran- sition. Injection of EGTA in calcium- free medium blocked anaphase and the gradual Ca i rise, but both features reappeared on addition of calcium to the medium. Nocodozole-induced depolymerization of the spindle apparatus also reversibly blocked these two events but had no effect on the transient changes. Hence the anaphase transition, although depen- dent on calcium (either intra- or extracellular) is not triggered by tran- sient increases in calcium levels.

Molecular genetic approach to the characterization of the 'Down

syndrome region' of chromosome 21 M.K. MCCORMICK ET AL.

Genomics 5, 325-331

The search for the Down syndrome (DS) gene is, unforttmately, not as advanced as that for the CF locus (see above). In the 30 years since the defects were linked to trisomy 21, the locus has been narrowed down to band 21q22 but little further. Taking advantage of molecular probes to single copy DNA sequences previously

mapped to points along chromosome 21, McCormick et al. have applied two molecular techniques - polymorphism analysis and dosage analysis - to determine the number of copies of these sequences in DS patients and non-DS patients with partial trisomy 21. The 'minimal chromosomal region' thus defined (the region containing all the loci trisomic only in DS sufferers) may need to be revised when probes become available in the 'bare' DNA stretches bordering the MCR. A chro- mosome 21-specific yeast artificial chromosome library should generate a

cdc2 and the regulation of mitosis: six interacting mcs genes

L. MOLZ, R. BOOHF~, P. YOUNG AND

D. BEACH

Genetics 122, 773-782

The protein kinase p34 cdc2, the prod- uct of the cdc2 + gene, controls initia- tion of mitosis in a wide range of eukaryotic cells. In fission yeast, p34 cdc2 is independently regulated by an activator, cdc25 ÷, and an inhibitor, wee1 +. It is possible to isolate muta- tions (wee mutations) that bypass either of these controls; in these mutants mitosis is advanced and cells divide at a small size. In double

mutants in which both control path- ways are circumvented, a lethal phenotype called mitotic catastrophe results, presumably because mitosis has initiated too early. Molz et al. have identified a number of mutations (mcs mutations) that suppress the lethality of mitotic catastrophe double mutants. They have uncovered a num- ber of interactions between the mcs mutations and mutations in other ele- ments that control mitosis in fission yeast, including cdc2 +, cdc25 ÷ and wee1 ÷ (see above). They propose that these mutations identify novel control elements that may be activators or substrates of the p34 cdc2 kinase. /~

Segment-specific expression of a homeobox-containing gene in the

mouse hindbrain P. MURPHY, D.R. DAVIDSON AND R.E. HILL

Nature341, 156-159

Homeotic genes have been implicated in many aspects of segment identity determination in Drosophila. In con- trast, vertebrate homeobox-containing genes have not previously been shown to follow any segmental pat- tern. One of the places where verte- brate morphological segmentation appears to be developmentally impor- tant is in the neuromeres of the central nervous system (CNS), on the basis of the pattern of nerve formation in the chick and the expression of the zinc-finger-containing gene Krox 20 in alternate segments in the mouse. This idea is reinforced by Murphy et al.,

who found that expression of the homeobox-containing gene Hox 2.9 is restricted to a single neuromere (rhombomere 4) in the developing mouse hindbrain; the limits of expres- sion are flanked on either side by domains expressing Krox 20. The restricted expression of Hox 2.9 sug- gests that the gene may be involved in determining segment identity, an idea that gains further credence from the fact that it closely resembles the Drosophila gene labial (part of the Antennapedia complex), which de- fines a single ancestral segment in the Drosophila head. It will be important to discover the boundaries of expres- sion of other homeobox genes within the developing CNS, to see if genes or combinations of genes are involved in achieving anteroposterior patterning in the mouse and, if so, how.

uniform series of probes across this region. Strangely, some cytogenetically diagnosed DS patients were not tri- somic for chromosome 21, according to the molecular criteria, suggesting that their cytogenetic abnormalities are either not in this chromosome, or involve regions not covered by probes. In addition, some DS patients with no obvious cytogenetic lesions showed no evidence of molecular tri- somy, suggesting that other factors are causing DS in these patients (or, again, that the abnormality is in one of the 'bare' regions).

Megabase-sized linear DNA in the bacterium Borrelia burgdorferi, the

Lyme disease agent

M.S. FERDOWS AND A.G. BARBOUR

Proc. Natl Acad. Sci. USA 86, 5969-5973

Although spirochetes have been given their own phylum (on the basis of their structure and ribosomal RNA sequences), and are responsible for disorders as important as, for example, Lyme disease, they remain poorly defined genetically. Ferdows and Barbour have taken the first step towards characterizing the genome of the spirochete responsible for Lyme disease - Borrelia burgdor fer i - by analysing the movement of its DNA on pulsed-field gels. Surprisingly, B. burgdorferi appears to have a linear chromosome, about 950 kb long, rather than the more usual circular chromosome found in virtually all prokaryotic cells (one of the unusual properties of the chromosome is its readiness to enter pulsed field gels, something that circular chromosomes do only very poorly). While it is possible that the linear molecule is simply a sheared circular chromo- some, the fact that the gel bands were sharp, and were in similar positions when different lysis procedures were used, argues other- wise. It will be interesting to discover if other Borrelia species (or other prokaryotes) have linear chromo- somes.

"FIG NOVEMBER 1989 VOL. 5 NO. 11

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