MoSNF1 regulates sporulation and pathogenicity in the rice blast fungus Magnaporthe oryzae

The protein kinase Snf1 is a major component of the glucose derepression pathway in yeast and a regulator of gene expression for the cell wall degrading enzyme (CWDE) in some plant pathogenic fungi. To address the molecular function of Snf1 in Magnaporthe oryzae, which causes the rice blast disease, MoSNF1 was cloned and functionally characterized using gene knock-out strategies. MoSNF1 functionally complemented the growth defect of the yeast snf1 mutant on a non-fermenting carbon source. However, the growth rate of the Δmosnf1 mutant on various carbon sources was reduced independent of glucose, and the expression of the CWDE genes in the mutant was induced during derepressing condition like the wild type. The pre-penetration stage including conidial germination and appressorium formation of the Δmosnf1 was largely impaired, and the pathogenicity of the Δmosnf1 was significantly reduced. Most strikingly, the Δmosnf1 mutant produced only a few conidia and had a high frequency of abnormally shaped conidia compared to the wild type. Our results suggest that MoSNF1 is a functional homolog of yeast Snf1, but its contribution to sporulation, vegetative growth and pathogenicity is critical in M. oryzae.     

Identification of repressor binding sites controlling expression of tetracycline resistance encoded by Tn10

The regulatory region controlling the expression of tetracycline resistance and repressor genes contains two nearly identical regions of dyad symmetry. Deletions of this control region were isolated by digestion with S1 nuclease. The ability of these deletions to bind the tet repressor was determined by an in vivo repressor titration assay. The results indicate that repressor specifically binds both regions of dyad symmetry.     

An activator/repressor dual system allows tight tetracycline-regulated gene expression in budding yeast.

We have developed an activator/repressor expression system for budding yeast in which tetracyclines control in opposite ways the ability of tetR-based activator and repressor molecules to bind tetO promoters. This combination allows tight expression of tetO- driven genes, both in a direct (tetracycline-repressible) and reverse (tetracycline-inducible) dual system. Ssn6 and Tup1, that are components of a general repressor complex in yeast, have been tested for their repressing properties in the dual system, using lacZ and CLN2 as reporter genes. Ssn6 gives better results and allows complete switching-off of the regulated genes, although increasing the levels of the Tup1-based repressor by expressing it from a stronger promoter improves repressing efficiency of the latter. Effector-mediated shifts between expression and non-expression conditions are rapid. The dual system here described may be useful for the functional analysis of essential genes whose conditional expression can be tightly controlled by tetracyclines.     

PdSNF1, a sucrose non-fermenting protein kinase gene, is required for Penicillium digitatum conidiation and virulence

The sucrose non-fermenting protein kinase 1 gene (SNF1) regulates the derepression of glucose-repressible genes in microorganisms. In this study, we cloned an ortholog of SNF1 from Penicillium digitatum and characterized its functions through a gene knock-out strategy. Growth of the PdSNF1 mutant (ΔPdSNF1) on the synthetic medium (SM) supplemented with pectin or polygalacturonic acid was severely disturbed. The appearance of disease symptoms on the ΔPdSNF1 mutant-inoculated citrus fruits was significantly delayed as well. The expression levels of the cell wall-degrading enzyme (CWDE) genes (e.g., XY1, PL1, PNL1, and EXPG2) after pectin induction were up-regulated in wild type, but unchanged or less up-regulated in the ΔPdSNF1 mutant. During infection in citrus fruit, the up-regulation of XY1 was delayed in the ΔPdSNF1 mutant. Disruption of PdSNF1 also resulted in impaired conidiation and caused malformation of the conidiophore structures. In addition, the expression of BrlA, a gene that regulates conidiophore development, was significantly impaired in the ΔPdSNF1 mutant. However, the expression of FadA, encoding the α-subunit of a heterotrimeric G protein, was up-regulated in this mutant. Collectively, our results demonstrate that the PdSNF1 plays a role in adapting P. digitatum to alternative carbon sources. Its involvements in the virulence of P. digitatum is probably via regulation of the expression of CWDE genes; and it is also involved in conidiation, probably through activation of the conidiation signaling pathway while inactivating the mycelial growth-signaling pathway.     

Effect of alkylating agents on the expression of inducible genes of Escherichia coli

Increasing doses of alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine, diethyl sulphate and ethylmethane sulphonate cause an inhibition of the expression of the recA and sfiA genes of wild-type Escherichia coli. This behaviour was not observed in a lexA56 mutant which has a defective LexA repressor that is unable to bind to the SOS operator. Furthermore, an ada-1 mutant showed the same behaviour as the wild-type strain indicating that the adaptive proteins are not responsible for the inhibition of recA and sfiA at high doses of alkylating agents. These results suggest that the inhibitory effect of these alkylating agents may be found in the interaction between the LexA repressor and the control regions of sfiA and recA. On the other hand, high doses of either UV light or mitomycin C produced only a slight decrease in the induction of recA and sfiA, whereas bleomycin had no effect. The fact that a repressor structurally related to LexA repressor, such as LacI protein, showed the same behaviour as the LexA repressor when a Lac+ strain was treated with alkylating agents, suggests that these compounds can modify the binding abilities of repressors to DNA, producing a limited or even abolished release of repressors, and so decreasing the expression of inducible genes.