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Roles for Sigma Factors in Global Circadian Regulation of the Cyanobacterial Genome. Usha Nair, 2002.The circadian clock of the unicellular cyanobacterium Synechococcus elongatus PCC 7942 imposes a global rhythm of transcription on promoters throughout the genome . Inactivation of any of the four known group 2 sigma factor genes (rpoD2, rpoD3, rpoD4, and sigC), singly or pairwise, altered circadian expression from the psbAI promoter, changing amplitude, phase angle, waveform, or period . However, only the rpoD2 mutation and the rpoD3 rpoD4 and rpoD2 rpoD3 double mutations affected expression from the kaiB promoter . A striking differential effect was a 2-h lengthening of the circadian period of expression from the promoter of psbAI, but not of those of kaiB or purF, when sigC was inactivated . The data show that separate timing circuits with different periods can coexist in a cell . Overexpression of rpoD2, rpoD3, rpoD4, or sigC also changed the period or abolished the rhythmicity of PpsbAI expression, consistent with a model in which sigma factors work as a consortium to convey circadian information to downstream genes . Novel Two-Component Regulatory System Involved in Biofilm Formation and Acid Resistance in Streptococcus mutans. Yung-Hua Li, 2002.The abilities of Streptococcus mutans to form biofilms and to survive acidic pH are regarded as two important virulence determinants in the pathogenesis of dental caries . Environmental stimuli are thought to regulate the expression of several genes associated with virulence factors through the activity of two-component signal transduction systems . Yet, little is known of the involvement of these systems in the physiology and pathogenicity of S . mutans . In this study, we describe a two-component regulatory system and its involvement in biofilm formation and acid resistance in S . mutans . By searching the S . mutans genome database with tblastn with the HK03 and RR03 protein sequences from S . pneumoniae as queries, we identified two genes, designated hk11 and rr11, that encode a putative histidine kinase and its cognate response regulator . To gain insight into their function, a PCR-mediated allelic-exchange mutagenesis strategy was used to create the hk11 (Emr) and rr11 (Emr) deletion mutants from S . mutans wild-type NG8 named SMHK11 and SMRR11, respectively . The mutants were examined for their growth rates, genetic competence, ability to form biofilms, and resistance to low-pH challenge . The results showed that deletion of hk11 or rr11 resulted in defects in biofilm formation and resistance to acidic pH . Both mutants formed biofilms with reduced biomass (50 to 70% of the density of the parent strain) . Scanning electron microscopy revealed that the biofilms formed by the mutants had sponge-like architecture with what appeared to be large gaps that resembled water channel-like structures . The mutant biofilms were composed of longer chains of cells than those of the parent biofilm . Deletion of hk11 also resulted in greatly diminished resistance to low pH, although we did not observe the same effect when rr11 was deleted . Genetic competence was not affected in either mutant . The results suggested that the gene product of hk11 in S . mutans might act as a pH sensor that could cross talk with one or more response regulators . We conclude that the two-component signal transduction system encoded by hk11 and rr11 represents a new regulatory system involved in biofilm formation and acid resistance in S . mutans . The pqrAB Operon Is Responsible for Paraquat Resistance in Streptomyces coelicolor. You-Hee Cho, 2003.Paraquat (methyl viologen)-resistant mutants of Streptomyces coelicolor A3(2) that grew and sporulated normally in the presence of paraquat were isolated . Based on the positions of the mutant loci in the genetic map, we isolated the pqr (paraquat resistance) gene whose mutation (pqr501) caused a dominant paraquat-resistant phenotype . The pqr locus consists of two genes (pqrA and pqrB) that form a transcription unit . The pqrA gene encodes a protein with a TetR-like DNA-binding motif, and the pqrB gene encodes a putative efflux pump of the major facilitator superfamily . The pqr501 mutation was a base substitution changing arginine-18 to glutamine (R18Q) near the helix-turn-helix motif in PqrA . A pqrA null mutant exhibited similar paraquat resistance, and an increase in the amount of pqrA promoter-driven transcripts of about eightfold was observed for the pqrA501 mutant . These results suggest that PqrA is a negative regulator of its own operon . Deletion of the pqrAB operon caused cells to be very sensitive to paraquat, consistent with the prediction that PqrB may function as a paraquat-efflux pump . Purified PqrA protein specifically bound to the pqrA promoter region, whereas mutant R18Q protein did not, indicating that PqrA is a direct autoregulator of its own operon . Stereoselective Microbial Dehalorespiration with Vicinal Dichlorinated Alkanes. Stefaan De Wildeman, 2003.The suspected carcinogen 1,2-dichloroethane (1,2-DCA) is the most abundant chlorinated C2 groundwater pollutant on earth . However, a reductive in situ detoxification technology for this compound does not exist . Although anaerobic dehalorespiring bacteria are known to catalyze several dechlorination steps in the reductive-degradation pathway of chlorinated ethenes and ethanes, no appropriate isolates that selectively and metabolically convert them into completely dechlorinated end products in defined growth media have been reported . Here we report on the isolation of Desulfitobacterium dichloroeliminans strain DCA1, a nutritionally defined anaerobic dehalorespiring bacterium that selectively converts 1,2-dichloroethane and all possible vicinal dichloropropanes and -butanes into completely dechlorinated end products . Menaquinone was identified as an essential cofactor for growth of strain DCA1 in pure culture . Strain DCA1 converts chiral chlorosubstrates, revealing the presence of a stereoselective dehalogenase that exclusively catalyzes an energy-conserving anti mechanistic dichloroelimination . Unlike any known dehalorespiring isolate, strain DCA1 does not carry out reductive hydrogenolysis reactions but rather exclusively dichloroeliminates its substrates . This unique dehalorespiratory biochemistry has shown promising application possibilities for bioremediation purposes and fine-chemical synthesis .
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