藤原 誠

Development of plastids into chloroplasts, the organelles of photosynthesis, is triggered by light. However, little is known of the factors involved in the complex coordination of light-induced plastid gene expression, which must be directed by both nuclear and plastid genomes. We have isolated an Arabidopsis mutant, abc1, with impaired chloroplast development, which results in a pale green leaf phenotype. The mutated nuclear gene encodes a sigma factor, SigB, presumably for the eubacterial-like plastid RNA polymerase. Our results provide direct evidence that a nuclear-derived prokaryotic-like SigB protein, plays a critical role in the coordination of the two genomes for chloroplast development.

Three new nuclear genes (sigD, sigE and sigF) of Arabidopsis thaliana, encoding putative plastid RNA polymerase sigma factors, were identified and analyzed. Phylogenetic analysis revealed that higher plant sigma factors fell into at least four distinct subgroups within a diverse protein family. In addition, Arabidopsis sig genes contained conserved chromosomal intron sites, indicating that these genes arose by DNA duplication events during plant evolution. Transcript analyses revealed two alternatively spliced transcripts generated from the sigD region, one of which is predicted to encode a sigma protein lacking the carboxy-terminal regions 3 and 4. Finally, the amino-terminal sequence of the sigF gene product was shown to function as a plastid-targeting signal using green fluorescent protein fusions.

Although many chitin synthase genes have been identified in a broad range of fungal species, there have been only a few reports about their role in fungal morphogenesis, In most cases, single gene disruption or replacement did not reveal their function, possibly because of functional redundancy among them. We obtained null mutants of Aspergillus nidulans chsA and chsC genes encoding non-essential class II and class I chitin synthases, respectively. The Delta chsA Delta chsC mutant exhibited growth defects on media supplemented with sodium dodecyl sulfate (SDS), high concentration of salts, chitin-binding dyes, or chitin synthase competitive inhibitors, suggesting loss of integrity of hyphal wall. Moreover, remarkable abnormalities of the double mutant were observed microscopically during its asexual development. The conidiophore population was drastically reduced, Interestingly, secondary conidiophores were occasionally produced from vesicles of the primary ones, The morphology of these conidiophores was similar to those of the A. nidulans developmental mutants, medusa (medA), abacus (abaA), and some kinds of bristle (brlA), lit situ staining patterns suggested that chsA was mainly expressed in the metulae, phialides, and conidia, whereas chsC was expressed in hyphae as well as conidiophores, These results suggest that ChsA and ChsC share critical functions in hyphal wall integrity and differentiation.

In plant cells, plastid DNA is transcribed by at least two types of RNA polymerase, plastid-encoded RNA polymerase (PEP) and nuclear-encoded RNA polymerase (NEP). PEP is homologous to eubacterial transcription machinery, but its regulatory subunit, sigma (sigma) factor, is not encoded on the plastid DNA. We previously cloned the three nuclear-encoded sigma factor genes from Arabidopsis thaliana and designated them as sigA, sigB, and sigC. By means of RFLP mapping, sigA and sigB were mapped on chromosome I and sigC on the chromosome III. Based on comparison of the genomic structure of the three sig genes, intron sites in the 3' half of the genes were shown to be identical between sigB and sigC but divergent in sigA, consistent with the phylogenetic relevance of the three gene products. A transient expression assay of GFP fusions in Arabidopsis protoplasts showed that the N-termini of all three sig gene products functioned as chloroplast-targeting signals. We also constructed transgenic Arabidopsis lines harboring the sigA-promoter or the sigB-promoter uidA fusion. Both the sigA- and sigB-promoters were similarly activated at cotyledons, hypocotyls, rosette leaves, cauline leaves, sepals, and siliques but not at roots, seeds, or other flower organs. In addition, the two promoters were repeatedly activated in young seedlings under continuous light, possibly in an oscillated fashion.

We have found that the Aspergillus nidulans csmA gene encodes a novel protein which consists of an N-terminal myosin motor-like domain and a C-terminal chitin synthase domain (M. Fujiwara, H. Horiuchi, A. Ohta, and M. Takagi, Biochem. Biophys. Res. Commun, 236:75-78, 1997). To clarify the roles of csmA in fungal morphogenesis, we constructed csmA null mutants. The growth rate of the mutant colonies was almost the same as that of the wild-type strain, but hyphal growth was severely inhibited when a chitin-binding reagent, Calcofluor white or Congo red, was added to the medium. Moreover, morphological abnormalities in tip growth and septum formation were identified microscopically. Proliferation of intracellular new hyphae, called intra-hyphal hyphae, which behaved as intrinsic hyphae, was the most striking phenotypic feature among them. These phenotypes were not suppressed when the only chitin synthase domain of csmA was expressed under the control of the alcA promoter, whereas they were suppressed when the intact form of csmA was expressed. Therefore, it was concluded that the product of csmA (CsmA) has important roles in polarized cell wall synthesis and maintenance of cell wall integrity and that the myosin motor-like domain is indispensable for these functions.

A csmA gene that encodes chitin synthase with a myosin motor-like domain was isolated from the filamentous fungus Aspergillus nidulans, Initially, we obtained the csmA as a homolog of the Aspergillus fumigatus chsE-partial fragment, A large open reading frame encoding a polypeptide of 1,852 a.a. was identified by determining the cDNA sequences, The chitin synthase conserved region was situated at the C-terminus and classified into class V as reported previously. On the other hand, the N-terminal region showed significant similarity to myosin motors and could not be classified into any types of myosins identified so far, Thus, it is suggested that this is the first report of unconventional myosin fused to a metabolic enzyme. The finding of this new type of chitin synthase gene suggests that localization of chitin synthesis may be guided by association with cytoskeletal structures. (C) 1997 Academic Press.

We previously isolated three chitin synthase genes (chsA, chsB, and chsC) from Aspergillus nidulans. In the present work, we describe the isolation and characterization of another chitin synthase gene, named chsD, from A. nidulans. Its deduced amino acid sequence shows 56.7% and 55.9% amino acid identity, respectively, with Call of Saccharomyces cerevisiae and Chs3 of Candida albicans. Disruption of chsD caused no defect in cell growth or morphology during the asexual cycle and caused no decrease in chitin content in hyphae. However, double disruption of chsA and chsD caused a remarkable decrease in the efficiency of conidia formation, while double disruption of chsC and chsD caused no defect. Thus it appears that chsA and chsD serve redundant functions in conidia formation.