In U. maydis the multiallelic b locus controls sexual and pathogenic development. In the b locus a gene coding for a regulatory protein had been identified, and it was suggested that the interaction of two b polypeptides specified by different alleles programs sexual development in this fungus. We now demonstrate the existence of a second regulatory gene in the b locus. We term this gene bW and refer to the former as the bE gene. Both genes exist in many alleles. Although unrelated in primary sequence, both genes are similar in their overall organization. The gene products display allele-specific variability in their N-terminal domains, show a high degree of sequence conservation in the C-terminal domains, and contain a homeodomain-related motif. Genetic evidence is provided to show that the pair of bE and bW polypeptides encoded by different b alleles is the key regulatory species.

SummaryNucleobases and derivatives like cytokinins and caffeine are translocated in the plant vascular system. Transport studies in cultured Arabidopsis cells indicate that adenine and cytokinin are transported by a common H -coupled high-af®nity purine transport system. Transport properties are similar to that of Arabidopsis purine transporters AtPUP1 and 2. When expressed in yeast, AtPUP1 and 2 mediate energydependent high-af®nity adenine uptake, whereas AtPUP3 activity was not detectable. Similar to the results from cell cultures, purine permeases (PUP) mediated uptake of adenine can be inhibited by cytokinins, indicating that cytokinins are transport substrates. Direct measurements demonstrate that AtPUP1 is capable of mediating uptake of radiolabeled trans-zeatin. Cytokinin uptake is strongly inhibited by adenine and isopentenyladenine but is poorly inhibited by 6-chloropurine. A number of physiological cytokinins including trans-and cis-zeatin are also ef®cient competitors for AtPUP2-mediated adenine uptake, suggesting that AtPUP2 is also able to mediate cytokinin transport. Furthermore, AtPUP1 mediates transport of caffeine and ribosylated purine derivatives in yeast. Promoter±reporter gene studies point towards AtPUP1 expression in the epithem of hydathodes and the stigma surface of siliques, suggesting a role in retrieval of cytokinins from xylem sap to prevent loss during guttation. The AtPUP2 promoter drives GUS reporter gene activity in the phloem of Arabidopsis leaves, indicating a role in long-distance transport of adenine and cytokinins. Promoter activity of AtPUP3 was only found in pollen. In summary, three closely related PUPs are differentially expressed in Arabidopsis and at least two PUPs have properties similar to the adenine and cytokinin transport system identi®ed in Arabidopsis cell cultures.

The BH3-only BID (BH3-interacting domain death agonist) protein has a critical function in the death-receptor pathway in the liver by triggering mitochondrial outer membrane permeabilization (MOMP). Here we show that MTCH2/MIMP (mitochondrial carrier homologue 2/Met-induced mitochondrial protein), a novel truncated BID (tBID)-interacting protein, is a surface-exposed outer mitochondrial membrane protein that facilitates the recruitment of tBID to mitochondria. Knockout of MTCH2/MIMP in embryonic stem cells and in mouse embryonic fibroblasts hinders the recruitment of tBID to mitochondria, the activation of Bax/Bak, MOMP, and apoptosis. Moreover, conditional knockout of MTCH2/MIMP in the liver decreases the sensitivity of mice to Fas-induced hepatocellular apoptosis and prevents the recruitment of tBID to liver mitochondria both in vivo and in vitro. In contrast, MTCH2/MIMP deletion had no effect on apoptosis induced by other pro-apoptotic Bcl-2 family members and no detectable effect on the outer membrane lipid composition. These loss-of-function models indicate that MTCH2/MIMP has a critical function in liver apoptosis by regulating the recruitment of tBID to mitochondria.

SummaryTo elucidate the role of NH 4 + transporters in N nutrition of tomato, two new NH 4 + transporter genes were isolated from cDNA libraries of root hairs or leaves of tomato. While LeAMT1;2 is closely related to LeAMT1;1 (75.6% amino acid identity), LeAMT1;3 is more distantly related (62.8% identity) and possesses two short upstream open reading frames in the 5¢ end of the mRNA and a particularly short N-terminus of the protein as unique features. When expressed in yeast mutants defective in NH 4 + uptake, all three genes complemented NH 4 + uptake. In roots of hydroponically grown plants, transcript levels of LeAMT1;2 increased after NH 4 + or NO 3 ± supply, while LeAMT1;1 was induced by N de®ciency coinciding with low glutamine concentrations, and LeAMT1;3 was not detected. In aeroponic culture, expression of LeAMT1;1 and LeAMT1;2 was higher in root hairs than in the remaining root fraction. Growth of plants at elevated CO 2 slightly decreased expression of LeAMT1;2 and LeAMT1;3 in leaves, but strongly repressed transcript levels of chloroplast glutamine synthetase and photorespiratory serine hydroxymethyl-transferase. Expression of LeAMT1;2 and LeAMT1;3 showed a reciprocal diurnal regulation with highest transcript levels of LeAMT1;3 in darkness and highest levels of LeAMT1;2 after onset of light. These results indicate that in tomato at least two high-af®nity NH 4 + transporters, LeAMT1;1 and LeAMT1;2, are differentially regulated by N and contribute to root hair-mediated NH 4 + acquisition from the rhizosphere. In leaves, the reciprocally expressed transporters LeAMT1;2 and LeAMT1;3 are supposed to play different roles in N metabolism, NH 4 + uptake and/or NH 3 retrieval during photorespiration.