Factors affecting the mRNA levels for the non-heme iron and effector-binding subunits of ribonucleotide reductase

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

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.

Regulation of deoxycytidine kinase activity and inhibition by DFDC

(1) Deoxycytidine kinase activity increased in a transformation- and progression-linked fashion in rat hepatomas of different proliferation rates. The activity also increased and was growth rate-linked in a series of tissue culture cell lines of human and animal tumors. (2) Deoxycytidine kinase activity was stringently linked with expression of the neoplastic proliferative program as it sharply increased in log phase in tissue culture cells of hepatoma 3924A and several human carcinoma strains. (3) Deoxycytidine kinase is subject to nutritional and hormonal regulation. On starvation the activity in liver decreased and on refeeding it returned to normal. Steroid hormone increased liver enzymic activity. Deoxycytidine kinase is substrate-inducible, since deoxycytidine injections in rat led to a 2- to 3-fold increase in hepatic enzyme activity. (4) Actinomycin or cycloheximide treatment blocked the increase in liver deoxycytidine kinase activity induced by steroid or deoxycytidine treatment. Therefore, it is assumed that the rise in deoxycytidine kinase activity requires new RNA and protein synthesis. (5) Cycloheximide treatment of rats carrying hepatomas yielded a t1/2 = 3.4 hr in the tumor for deoxycytidine kinase activity which was the shortest among the examined enzymes of purine and pyrimidine biosynthesis. (6) Actinomycin treatment of rats carrying hepatomas yielded a t1/2 of 5.8 hr for deoxycytidine kinase activity in the tumor which was one of the shortest in the examined enzymes of purine and pyrimidine biosynthesis. (7) Difluorodeoxycytidine (DFDC) is a competitive inhibitor (Ki = 7-28 microM) of deoxycytidine kinase from rat hepatoma and from human pancreatic carcinoma and ovarian carcinoma cells in culture.