The murine long-term multi-lineage renewal marrow stem cell is a cycling cell.

Prevailing wisdom holds that hematopoietic stem cells (HSCs) are predominantly quiescent. Although HSC cycle status has long been the subject of scrutiny, virtually all marrow stem cell research has been based on studies of highly purified HSCs. Here we explored the cell cycle status of marrow stem cells in un-separated whole bone marrow (WBM). We show that a large number of long-term multi-lineage engraftable stem cells within WBM are in S/G2/M phase. Using bromodeoxyuridine, we show rapid transit through the cell cycle of a previously defined relatively dormant purified stem cell, the long-term HSC (LT-HSC; Lineage(-)/c-kit(+)/Sca-1(+)/Flk-2(-)). Actively cycling marrow stem cells have continually changing phenotype with cell cycle transit, likely rendering them difficult to purify to homogeneity. Indeed, as WBM contains actively cycling stem cells, and highly purified stem cells engraft predominantly while quiescent, it follows that the population of cycling marrow stem cells within WBM are lost during purification. Our studies indicate that both the discarded lineage-positive and lineage-negative marrow cells in a stem cell separation contain cycling stem cells. We propose that future work should encompass this larger population of cycling stem cells that is poorly represented in current studies solely focused on purified stem cell populations.

BaeI, another unusual BcgI-like restriction endonuclease.

BcgI and BcgI-like restriction endonucleases have a very distinct characteristic which causes them to differ from the other classified restriction enzymes; they all cleave double-stranded DNA specifically on both sides of the recognition sequence to excise a short DNA fragment including the recognition sites. Here we report a new BcgI-like restriction endonuclease, BaeI, isolated from Bacillus sphaericus. Like BcgI, BaeI also cleaves double-stranded DNA on both strands upstream and downstream of its recognition sequence (10/15)ACNNNNGTAYC(12/7). There are two dominant polypeptides in the final preparation of BaeI with molecular masses of approximately 80 and 55 kDa. Both are slightly larger than the two BcgI subunits. BaeI requires both Mg2+ and AdoMet to cleave DNA. Accompanying bilateral cleavage activity, the heteromeric BaeI also has an N6-adenine methyltransferase activity which modifies the symmetrically located adenines within its recognition sequence.

Thermostable Bst DNA polymerase I lacks a 3'-->5' proofreading exonuclease activity.

A thermostable DNA polymerase, the Bst DNA polymerase I, from Bacillus stearothermophilus N3468 was prepared to near-homogeneity. The dominant species of the Bst DNA polymerase I preparation sized about 97 kDa when analyzed on SDS polyacrylamide gels. The Bst polA gene that codes for Bst polymerase I was cloned and sequenced. Comparative sequence analysis showed that all three conserved 3'-->5' exonuclease motifs found in E. coli DNA polymerase I were missing in Bst DNA polymerase I. This cast doubt on the existence of a 3'-->5' exonuclease function in that enzyme. Four biochemical assays were used to measure exonuclease activities of Bst DNA polymerase I, testing both full-length Bst polymerase I and the Bst large fragment which lacks the N-terminal 5'-->3' exonuclease domain. These exonuclease assays demonstrated that Bst DNA polymerase I only contained a double-strand dependent 5'-->3' exonuclease activity but lacked any detectable 3'-->5' proofreading exonuclease activity. The lack of 3'-->5' exonuclease function in a variety of thermostable repair DNA polymerases may reflect enhancement of thermostability at the expense of proofreading activity.