Sucrose synthase and RuBisCo expression is similarly regulated by the nitrogen source in the nitrogen-fixing cyanobacterium Anabaena sp

Abstract

In higher plants and cyanobacteria, sucrose (Suc) metabolism is carried out by a similar set of enzymes. The function and regulation of Sue metabolism in cyanobacteria has begun to be elucidated. In strains of Anabaena sp., filamentous nitrogen-fixing cyanobacteria, Sue synthase (SuS, EC 2.4.1.13) controls Sue cell level through the cleavage of the disaccharide. The present work shows that there are two sus genes in Anabaena (Nostoc) sp. that are co-regulated regarding the nitrogen source; however, only susA accounts for the extractable SuS activity and for the control of the Sue level. Primer extension analysis has uncovered the sequence of the Anabaena susA and susB ammonium-activated putative promoters, which share a high sequence similarity with that of rbcLS encoding ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.39) and other ammonium up-regulated genes. Moreover, susA and rbcLS expression is developmentally co-localized to the vegetative cells of the nitrogen-fixing cyanobacterial filaments. Our results strongly suggest the existence of a regulatory network that would coordinate the expression of key genes for Sue and nitrogen metabolism, carbon fixation, and development in Anabaena sp.