Biphasic expression of rnrB in Dictyostelium discoideum suggests a direct relationship between cell cycle control and cell differentiation

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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Cycle / genetics*
  • Cell Differentiation / genetics*
  • Cell Division / genetics
  • Cold Temperature
  • Cytological Techniques
  • Dictyostelium / cytology*
  • Dictyostelium / genetics*
  • Gene Expression Regulation, Developmental
  • Promoter Regions, Genetic
  • Ribonucleotide Reductases / genetics*


  • Ribonucleotide Reductases