MacWilliams H, Gaudet P, Deichsel H, Bonfils C, Tsang A
Zoologisches Institut, Ludwig-Maximilians-Universitat Luisenstrasse 14, 80333 München 2, Germany.
Differentiation. 2001 Feb;67(1-2):12-24. doi: 10.1046/j.1432-0436.2001.067001012.x.
Cell differentiation in Dictyostelium is strongly affected by the cell cycle. Cell cycle control is well-understood in other systems, but this has had almost no impact on the study of Dictyostelium cell differentiation, in part because the cell cycle in Dictyostelium is unusual, lacking a G1 phase. Here we describe the cell-cycle regulated expression of rnrB, which codes for the small subunit of ribonucleotide reductase and is a marker of late G1 in many systems. There appear to be two expression peaks, one in mid-G2 and the other near the G2/M transition. Using Xgal/anti-BrdU double staining, we show that cells in asynchronously growing cultures express in both phases, with a gap between them during which the gene is transcriptionally silent. Cold-synchronized cells show exclusively G2/M expression, while mid-G2 expression is seen in high-density synchronized cells and can also be inferred in cells undergoing synchronization by either method. rnrB expression occurs in other systems shortly after cells pass a point (the "restriction point" or "start") at which they commit to complete their current cell cycle. We demonstrate a similar commitment point in Dictyostelium and show that this occurs shortly before the mid-G2 rnrB expression peak. The Dictyostelium cell cycle thus appears to include a well-defined though inconspicuous event, between early and mid-G2, with some features which are normally associated with the G1/S transition. Others have described a switch from stalk to spore differentiation preference at about this time. Since Dictyostelium cells switch back from spore to stalk preference approximately at the G2/M rnrB expression maximum, cell differentiation as well as rnrB expression may be regulated directly by fundamental cell cycle control processes.
盘基网柄菌中的细胞分化受到细胞周期的强烈影响。细胞周期调控在其他系统中已得到充分理解，但这对盘基网柄菌细胞分化的研究几乎没有影响，部分原因是盘基网柄菌的细胞周期不同寻常，缺乏G1期。在这里，我们描述了rnrB的细胞周期调控表达，rnrB编码核糖核苷酸还原酶的小亚基，在许多系统中是G1晚期的标志物。似乎有两个表达峰值，一个在G2中期，另一个在G2/M转换附近。使用Xgal/抗BrdU双重染色，我们表明异步生长培养物中的细胞在两个阶段都有表达，它们之间有一个间隙，在此期间该基因转录沉默。冷同步化的细胞仅显示G2/M期表达，而G2中期表达则出现在高密度同步化的细胞中，并且通过任何一种方法进行同步化的细胞中也可以推断出这种表达。rnrB表达在其他系统中发生在细胞通过一个点（“限制点”或“起始点”）后不久，在该点它们承诺完成当前的细胞周期。我们在盘基网柄菌中证明了一个类似的承诺点，并表明这发生在G2中期rnrB表达峰值前不久。因此，盘基网柄菌的细胞周期似乎包括一个在G2早期和中期之间定义明确但不明显的事件，具有一些通常与G1/S转换相关的特征。其他人描述了大约在这个时候从柄分化偏好到孢子分化偏好的转变。由于盘基网柄菌细胞大约在G2/M期rnrB表达最大值时从孢子偏好切换回柄偏好，细胞分化以及rnrB表达可能直接受基本细胞周期控制过程的调节。
