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© The Rockefeller University Press, 0021-9525/2001/7/12-a $5.00
The Journal of Cell Biology, Volume 154, Number 1, July 9, 2001 12-a-12

Algal Rb sets size limits

Multicellularity in Volvox may have arisen from Rb's cell cycle tricks. David Kirk

Growth and division are uncoupled in Chlamydomonas cell cycles. During the day, cells remain in G1 to concentrate on photosynthesis, often growing to many times their original size. At night, however, cells undergo multiple alternating rounds of S phase and mitosis to produce 2ⁿ daughter cells.

Jim Umen and Ursula Goodenough of Washington University (St. Louis, MO) found that cells with a deletion in the mat3 gene of Chlamydomonas are smaller because of two perturbations in this cell cycle regimen. Mutant cells committed to entering a cell division cycle at a smaller than normal size, and those cells then underwent more than the usual number of rounds of DNA replication and division.

The gene at the mat3 locus encodes a protein that is very similar to mammalian Rb. This mammalian tumor suppressor restrains initiation of S phase when growth signals are absent. Mammalian Rb loss results in a shortened G1, whereas cells lacking mat3 retain their characteristically long G1, suggesting that the Chlamydomonas Rb pathway is not an S-phase switch, but a size sensor restraining cell cycle progression.

Umen notes that multicellular algal species often have 2ⁿ cells, suggesting that multicellularity arose when cells remained stuck together after a Chlamydomonas-style cell cycle. The Rb pathway, as the only Chlamydomonas pathway known to control cell size and the number of replication cycles, is a leading candidate for originally controlling the n in 2ⁿ.

William A. Wells

wells{at}rockefeller.edu

References

Umen, J., and U. Goodenough. 2001. Genes Dev. 15:1652–1661.[Abstract/Free Full Text]

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This Article PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Wells, W. A. Search for Related Content PubMed Articles by Wells, W. A. Social Bookmarking                            What's this?