By regulatory proteins directly involved in sensing intracellular pyrimidine concentrations. In this work, we have shown that mutations in genes belonging to de novo nucleotide biosynthetic pathways strongly affect csgDEFG expression and curli production in E. coli. Interplay between nucleotide metabolism and biofilm appears to be conserved in different bacteria; however, specific effects and mechanism may vary substantially. Indeed, although our results are consistent with PYR-41 previous findings showing that active de novo UMP biosynthesis is necessary for biofilm formation in P. aeruginosa, in this bacterium inhibition of purine biosynthesis through inactivation of the purH gene does not affect adhesion factors�� production, in contrast to what observed in E. coli. Likewise, pyrimidines appear to control EPS production and biofilm formation in V. cholerae through the dedicated regulator CytR, which does not appear to play a direct role in curli regulation in E. coli. Despite these differences, it seems that absence of de novo pyrimidine biosynthesis can act as a signal for severe nutrient starvation, which can in turn prevent biofilm formation and promote biofilm dispersal. In E. coli, the effects of mutations in the de novo UMP biosynthesis on curli production can be complemented by supplementing growth medium with uracil, thus suggesting that pyrimidine nucleotide availability, regardless whether it is achieved via de novo UMP biosynthesis or the pyrimidine salvage pathway, allows efficient csgDEFG transcription and expression of the CsgD regulon. Regulation of csgDEFG expression by intracellular nucleotide concentrations might take place by direct modulation of transcription initiation by RNA polymerase, similar to transcription control by GTP availability described for ribosomal promoters, or through not yet identified nucleotide-sensing regulatory proteins. Alternatively, perturbations in nucleotide pools might affect accumulation of cdi-GMP, a signal molecule necessary for csgDEFG expression, possibly by impairing Bulleyaconi-cine-A Diguanylate cyclases�� enzymatic activity. Diguanylate cyclases play a role in pyrimidine-dependent regulation of cellulose production. Cellulose production is regulated by a more complex mechanism since, in addition to pyrimidine availability, it seems to respond to the relative activity of the two UMP biosynthetic pathways. Indeed, MG1655 produces twice as much cellulose when grown in the presence of exogenous uracil, i.e., in conditions in which UMP biosynthesis is mostly carried out via the pyrimidine salvage pathway and de novo UMP biosynthesis is inhibited.