Constitutively high dNTP concentration inhibits cell cycle progression and the DNA damage checkpoint in yeast Saccharomyces cerevisiae

In eukaryotic cells the concentration of dNTP is highest in S phase and lowest in G(1) phase and is controlled by ribonucleotide reductase (RNR). RNR activity is eliminated in all eukaryotes in G(1) phase by a variety of mechanisms: transcriptional regulation, small inhibitory proteins, and protein degradation. After activation of RNR upon commitment to S phase, dATP feedback inhibition ensures that the dNTP concentration does not exceed a certain maximal level. It is not apparent why limitation of dNTP concentration is necessary in G(1) phase. In principle, dATP feedback inhibition should be sufficient to couple dNTP production to utilization. We demonstrate that in Saccharomyces cerevisiae constitutively high dNTP concentration transiently arrests cell cycle progression in late G(1) phase, affects activation of origins of replication, and inhibits the DNA damage checkpoint. We propose that fluctuation of dNTP concentration controls cell cycle progression and the initiation of DNA replication

Cyclical expression of L-selectin (CD62L) by recirculating T cells

L-Selectin (CD62L) mediates T-cell entry into lymph nodes. Whether the microenvironment modulates L-selectin expression of T cells during diapedesis and transit is unknown. Therefore, L-selectin expression was determined quantitatively on circulating T cells in blood, lymph nodes and thoracic duct by confocal laser scanning microscopy. We show that in contrast to leukocyte function-associated antigen-1 (CD11a/CD18) and ICAM-1 (CD54), L-selectin expression is cyclically expressed on recirculating T cells. It is reduced to ∼30% of the blood value during entry across high endothelial venules. Within lymph nodes, CD4+ T-cell subsets maintain reduced L-selectin expression at a similar level in all compartments (T-cell zone, B-cell zone and medulla). After exit, L-selectin is re-expressed to levels comparable to those of T cells in blood. Apparently, L-selectin levels are not only down-regulated during T-cell activation but also routinely reduced while transmigrating within lymph nodes. L-Selectin down-regulation seems to be ligand independent since it also occurs in the white pulp compartments of the spleen which lack classic L-selectin ligands such as GlyCAM-1 and CD34. In addition, T cells in non-lymphoid organs do not reveal reduced L-selectin levels. Thus, the ability of secondary lymphoid organs to reduce L-selectin expression of T cells prior to activation might be a prerequisite for their characteristic property to induce primary immune responses