Transcriptional analysis of Rhizobium tropici CIAT 899 guaB region

COLLAVINO M.M. Y AGUILAR, O.M.

Villa Carlos Paz, Córdoba

Congreso; Reunión Anual de la Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB); 2002

Mutant strain, R. tropici CIAT899-10.T (GuaB-) is defective in its intrinsic thermal tolerance and also in the interaction with P. vulgaris. The guaB gene encodes for the enzyme inosine-monophosphate-dehydrogenase of the guanine biosynthetic pathway. A chromosomal region containing the guaB gene was cloned and partially sequenced. Two additional open reading frames were identified adjacent to guaB: ORF1, upstream guaB, is homologous to a S. meliloti putative signal peptide. The downstream ORF (ORF2) has no homology in the data base. In order to study the transcription activity of sequences located upstream from the three ORFs, respectively, transcriptional fusions by using the reporter lacZ gene were constructed and assayed in both a R. tropici wild type and a guaB mutant background. In all conditions tested, the expression of the guaB gene was found to be higher than that of the flanking ORFs. Expression of guaB is two folds higher in mutant than wild type. Addition of guanine and xanthine to the medium decrease guaB expression after four hours, whereas transcription levels were reduced twenty hours after addition of adenine and hypoxanthine. The pattern of expression is not affected by thermal stress. ß-galactosidase level of the ORF3::lacZ fusion in both the wild type and mutant 10.T strains was similar. These findings suggest that the transcription of the guaB gene is negatively affected by the products of the GuaB activity (guanine and xanthine) and that transcription of the ORF 3 is independent of the guaB gene.