MucR and mucS activate exp genes transcription and galactoglucan production in Sinorhizobium meliloti EFB1.
When grown under standard conditions, Sinorhizobium meliloti EFB1 simultaneously produces two acidic exopolysaccharides, succinoglycan and galactoglucan, yielding very mucoid colonies. In this strain, MucR is essential for galactoglucan synthesis. A mutation in the mucS gene resulted in less mucoid colonies than in the wild-type EFB1. This mucS- strain was complemented to the wild-type phenotype by the cloned mucS gene, indicating that mucS is necessary for a wild-type level of galactoglucan production. Reverse transcription-polymerase chain reaction analysis of exp genes, which encode the pathway for galactoglucan production, in EFB1 and in the mutants affected in mucS, mucR, and both genes simultaneously, showed that MucS is a transcriptional activator of the exp genes but does not affect its own transcription. Furthermore, MucR is necessary for mucS transcriptional activation. As introduction of a cloned mucS gene in a mucR- strain yielded colonies less mucoid than the wild type, MucR could also activate exp genes transcription through other pathways. Deletion analysis of the expE promoter showed a region important for transcription and MucS activation. This region, containing a palindrome, is present in the putative expA, expC, expD, and expE promoters but not in the mucS promoter, suggesting that it is the target for MucS. A mucR-mucS- mutant, which does not produce galactoglucan, was impaired in competitive nodulation of alfalfa in soil microcosms, indicating another possible role for this exopolysaccharide in symbiosis.
Lloret
Javier
J
Departamento de Biología, Universidad Autónoma de Madrid, Spain.
Martín
Marta
M
Oruezabal
Roke I
RI
Bonilla
Ildefonso
I
Rivilla
Rafael
R
D001426:Bacterial Proteins / Q000378:metabolism*; D001483:Base Sequence; D003001:Cloning, Molecular; D017931:DNA Primers; D004926:Escherichia coli / Q000235:genetics; D005656:Fungal Proteins / Q000378:metabolism*; D005685:Galactans; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D005816:Genetic Complementation Test; D005936:Glucans; D016296:Mutagenesis; D011135:Polysaccharides, Bacterial / Q000235:genetics*; D012097:Repressor Proteins; D015183:Restriction Mapping; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D016962:Sinorhizobium meliloti / Q000235:genetics*; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors
D001426:Bacterial Proteins; D017931:DNA Primers; C100020:EXPG protein, Rhizobium meliloti; D005656:Fungal Proteins; D005685:Galactans; D005936:Glucans; C093770:MucR protein, Rhizobium meliloti; D011135:Polysaccharides, Bacterial; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors; C093768:galactoglucan