Bicarbonate-Mediated CO2 Formation on Both Sides of Photosystem II

The effect of bicarbonate (HCO3-) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO3- enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO2 and H2O. The other is for the electron-acceptor side, in which (i) reduction of the QA quinone leads to the release of HCO3- from its binding site on the non-heme iron and (ii) the Em potential of the QA/QA•- couple increases when HCO3- dissociates. This suggested a protective/regulatory role of HCO3- as it is known that increasing the Em of QA decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with 13C isotope labeling of HCO3- allows donor- and acceptor-side formation of CO2 by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.

Widespread B29 (CD79b) gene defects and loss of expression in chronic lymphocytic leukemia

Chronic Lymphocytic Leukemia (CLL) is the most prevalent form of leukemia in Western countries, and is characterized by a monoclonal proliferation of primarily immature CD5+ B lymphocytes. The molecular biology of chronic leukemias and lymphomas remains largely unresolved. Surface immunoglobulin (Ig) expression, which is often decreased in CLL, requires the protein product of the B29 gene for translocation of the B cell antigen receptor complex (BCR) to the cell surface and for signal transduction. Because B29 is essential for intracellular assembly and transport of the B cell antigen receptor complex to the cell surface, we postulate that a perturbation in B29 could result in the diminished expression and function of surface Ig in leukemic CLL cells. We have found recurrent aberrations affecting the B29 gene in CLL cells. Analyses of 27 unselected cases of CLL demonstrate that over 75% had low to absent B29 expression which correlated directly to their level of surface Ig expression. Half of these surface B29(low/-) cases had either no or barely detectable levels of B29 mRNA by RNAse protection assay. To date, all of the CLL samples with normal B29 mRNA levels have been found to have point mutations or truncations which would significantly effect the structure and/or function of B29 protein. Strategies directed at correcting these B29 mutations are expected to induce increased Ig surface expression in CLL and may improve the sensitivity of CLL cells to conventional cytotoxic chemotherapy.

Aberrations of the B-cell receptor B29 (CD79b) gene in chronic lymphocytic leukemia

Leukemic B cells in chronic lymphocytic leukemia (B-CLL) typically exhibit low or undetectable surface Ig. Because the B29 (CD79b and Ig beta) and mb-1 (CD79a and Ig alpha) gene products are required for surface Ig display in the B-cell receptor complex (BCR), we analyzed the expression of these genes in B-CLL cells. The majority (83%) of the randomly selected B-CLL patient samples analyzed exhibited low or undetectable surface BCR measured by mu heavy chain and B29 expression. Levels of mb-1 mRNA in these B-CLL samples with low surface BCR were similar to those in normal B cells. Among those with decreased surface expression, B29 mRNA was not detected in half of these B-CLL samples. The remaining B-CLL samples with diminished surface BCR contained normal levels of B29 mRNA. Further analysis of cDNA clones from the majority of these latter samples contained point mutations, insertions, or deletions that were largely located in the B29 transmembrane and cytoplasmic domains. These results indicate the occurrence of somatic mutations predicted to affect B29 expression and/or function in the majority of B-CLL and suggest that these aberrations underlie the diminished surface BCR display and loss of BCR signaling characteristic of this leukemia.

Cloning of the human homolog of the CDC34 cell cycle gene by complementation in yeast

In a screen designed to isolate human cDNAs that complement a yeast G2 phase checkpoint mutation (mec1), we isolated a cDNA homologous to the Saccharomyces cerevisiae CDC34 gene. The human CDC34 cDNA can functionally substitute for the yeast CDC34 gene and represents a mammalian homolog of the group of yeast genes required for the late G1-->S phase transition. The human CDC34 gene is expressed in multiple cell lines as a unique species and Southern blot analysis reveals evidence for a single gene that is highly conserved in higher eukaryotes. The human gene is located on the far telomeric region of 19p13.3 in a location that defines a region of homology between human chromosome 19p and mouse chromosome 11.