The problem of how fungal and oomycete avirulence proteins enter plant cells

Recent advances in cloning avirulence genes from a rust fungus and three oomycete species have provided the novel insight that these eukaryotic plant pathogens deliver small proteins into the host cell cytoplasm where they are recognized by resistance proteins. Anne Rehmany et al. have recently identified a potential host-targeting signal in oomycete avirulence proteins from Hyaloperonospora parasitica, Phytophthora sojae and Phytophthora infestans that might be involved in transporting proteins into the host cell. This signal is surprisingly similar to the host targeting signal used by the malaria pathogen Plasmodium fulciparum to target virulence proteins to the mammalian host cell.     

Pumpkin phloem lectin genes are specifically expressed in companion cells.

Pumpkin phloem exudate contains two abundant phloem proteins: PP1 is a 96-kD protein that forms polymeric filaments in vivo, and PP2 is a 48-kD dimeric lectin. Polyclonal antibodies raised against pumpkin phloem exudate were used to isolate several cDNAs corresponding to PP1 and PP2. RNA gel blot analysis indicated that PP1 is encoded by an mRNA of approximately 2500 nucleotides, whereas PP2 subunits are encoded by an mRNA of 1000 nucleotides. Sequence analysis of PP2 cDNAs revealed a 654-bp open reading frame encoding a 218-amino acid polypeptide; this polypeptide had the carbohydrate binding characteristics of a PP2 subunit. The PP2 mRNA was localized within the phloem of pumpkin hypocotyl cross-sections based on in situ hybridization of a digoxigenin-labeled antisense probe. PP2 mRNA was found within the companion cells in both the bicollateral vascular bundles and the extrafascicular phloem network.     

Two 5S genes are expressed in chicken somatic cells.

Two 5S RNA species were detected in chicken cells. 5S I RNA has the nucleotide sequence of chicken 5S RNA previously published by Brownlee et al. (1) and 5S II RNA differs from it by 10 mutations. The secondary structure of both species is compatible with that proposed for other eukaryotic 5S RNAs. 5S II RNA represents 50-60% of 5S I RNA. Both species were found in total chicken liver and brain and were present in polysomes in the same relative proportions. Only one 5S RNA species could be detected in rat liver and HeLa cells. Chicken is the first vertebrate described so far in which two 5S RNA genes are expressed in somatic cells.     

Multiple genes encoding zinc finger domains are expressed in human T cells

Proteins containing zinc finger domains have been implicated in developmental control of gene expression in Drosophila, Xenopus, mouse, and humans. Multiple cDNAs encoding zinc (II) finger structures were isolated from human cell lines of T-cell origin to explore whether zinc finger genes participate in the differentiation of human hematopoietic cells. Initial restriction analysis, genomic Southern blotting, and partial sequence comparisons revealed at least 30 nonoverlapping cDNAs designated cKox(1-30) encoding zinc finger motifs. Analysis of cKox1 demonstrated that Kox1 is a single-copy gene that is expressed in a variety of hematopoietic and nonhaematopoietic cell lines. cKox1 encodes 11 zinc fingers that were shown to bind zinc when expressed as a beta-gal-Kox1 fusion protein. Further analysis of the predicted amino acid sequence revealed a heptad repeat of leucines NH2-terminal to the finger region, which suggests a potential domain for homo- or heterodimer protein formation. On the basis of screening results it was estimated that approximately 70 zinc finger genes are expressed in human T cells. Zinc finger motifs are probably present in a large family of proteins with quite diverse and distinct functions. However, comparisons of individual finger regions in cKox1 with finger regions of cKox2 to cKox30 showed that some zinc fingers are highly conserved in their putative alpha-helical DNA binding region, supporting the notion of a zinc finger-specific DNA recognition code.     

The chloroplast-derived trnW and trnM-e genes are not expressed in Arabidopsis mitochondria

Depending on their genetic origin, plant mitochondrial tRNAs are classified into three categories: the "native" and "chloroplast-like" mitochondrial-encoded tRNAs and the imported nuclear-encoded tRNAs. The number and identity of tRNAs in each category change from one plant specie to another. As some plant mitochondrial trn genes were found to be not expressed, and as all Arabidopsis thaliana mitochondrial trn genes are known, we systematically tested the expression of A. thaliana mitochondrial trn genes. Both the "chloroplast-like" trnW and trnM-e genes were found to be not expressed. These exceptions are remarkable since trnW and trnM-e are expressed in the mitochondria of other land plants. Whereas we could not conclude which tRNA(Met) compensates the lack of expression of trnM-e, we showed that the cytosolic tRNA(Trp) is present in A. thaliana mitochondria, thus compensating the absence of expression of the mitochondrial-encoded trnW.     

The metastasis-associated genes MTA1 and MTA3 are abundantly expressed in human placenta and chorionic carcinoma cells

Normal placenta development relies on the ability of trophoblast cells to invade into the uterus and to build up an extensively vascularized feto-maternal tissue, necessary for the nutrition of the embryo. The ability of cell migration, invasion, and the ability to induce neovascularization are likewise hallmarks of cancer cells. The metastasis-associated genes MTA1 and MTA3 are known to be involved in cancer cell migration by regulation of cell adhesion proteins and to induce the expression of neoangiogenic cytokines, as recently shown by us for ovarian cancer cells. Therefore, we analyzed the expression of MTA1 and MTA3 in normal human placenta tissues and the chorionic cancer cell lines BeWo, JEG, and JAR. Immunohistochemical analysis revealed a rather strong expression of MTA1 and MTA3 in the nuclei of human trophoblast cells. A high expression level of MTA1 and MTA3 was further observed in the nuclei of human chorionic carcinoma cells, as shown by immunofluorescence analysis, and confirmed by Western blot and RT-PCR analysis. We conclude that the high expression level of MTA proteins in human chorionic cells might facilitate trophoblast cell migration and neoangiogenesis, and might further predispose human chorionic cancer cells with properties that are characteristic for this highly aggressive and metastatic carcinoma type.     

Tachykinin family genes and their receptors are differentially expressed in the hypothyroid ovary and pituitary.

Plasma tachykinin levels are known to be altered with sexual acyclicity and loss of reproductive function. Ovulatory dysfunction, as seen in postmenopausal women, is also often encountered in hypothyroid patients. To know the involvement of different tachykinin genes in hypothyroidism-associated reproductive disorders, we performed DD-PCR with the pituitary RNA of control and hypothyroid rats to see the differentially expressed gene profile. Subsequently, we selected a few clones, tachykinin being one of them. Since its expression was up regulated in hypothyroidism as it does in the sexually acyclic females, we wanted to correlate these two phenomena with hypothyroidism associated reproductive disorders. We observed differential expression of tac2 along with other tk genes and their receptors in rat pituitary and ovary, which suggests that hypothyroidism affects the expression of these genes in these tissues. The experiments were repeated in ovarian tissue obtained at surgery from hypothyroid human patients, which showed similar expression pattern of TAC3 (equivalent to rat tac2) and their receptors as in rat ovary. Significant reduction of tac2 expression in reproductively less active rat ovary suggests the association of tac2 with reproductive senescence. Our results suggest that decline in reproductive function in hypothyroidism is associated with altered expression level of tac2 and its receptors. Further investigation in this area could elucidate the possible mechanism of tachykinins' involvement in loss of sexual cyclicity and other reproductive disorders associated with hypothyroidism.