3 CDG OPERONS CONTROL CELLULAR-TURNOVER OF CYCLIC DI-GMP IN ACETOBACTER-XYLINUM - GENETIC ORGANIZATION AND OCCURRENCE OF CONSERVED DOMAINS IN ISOENZYMES

Cyclic di-GMP (c-di-GMP) is the specific nucleotide regulator of beta- 1,4-glucan (cellulose) synthase in Acetobacter xylinum. The enzymes co ntrolling turnover of c-di-GMP are diguanylate cyclase (DGC),which cat alyzes its formation, and phosphodiesterase A (PDEA), which catalyzes its degradation, Following biochemical purification of DGC and PDEA, g enes encoding isoforms of these enzymes have been isolated and found t o be located on three distinct yet highly homologous operons for cycli c diguanylate, cdg1, cdg2, and cdg3, Within each cdg operon, a pdeA ge ne lies upstream of a dgc gene. cdg1 contains two additional flanking genes, cdg1a and cdg1d. cdg1a encodes a putative transcriptional activ ator, similar to AadR of Rhodopseudomonas palustris and FixK proteins of rhizobia, The deduced DGC and PDEA proteins have an identical motif structure of two lengthy domains in their C-terminal regions, These d omains are also present in numerous bacterial proteins of undefined fu nction, The N termini of the DGC and PDEA deduced proteins contain put ative oxygen sensing domains, based on similarity to domains on bacter ial NifL and FixL proteins, respectively. Genetic disruption analyses demonstrated a physiological hierarchy among the cdg operons, such tha t cdg1 contributes 80% of cellular DGC and PDEA activities and cdg2 an d cdg3 contribute 15 and 5%, respectively. Disruption of dgc genes mar kedly reduced in vivo cellulose production, demonstrating that c-di-GM P controls this process.