Detection of two loci involved in (1-->3)-beta-glucan (curdlan) biosynthesis by Agrobacterium sp. ATCC31749, and comparative sequence analysis of the putative curdlan synthase gene

Genes essential for the production of a linear, bacterial (1-->3)-beta-glucan, curdlan, have been cloned for the first time from Agrobacterium sp.ATCC31749. The genes occurred in two, nonoverlapping, genomic fragmentsthat complemented different sets of curdlan( crd )-deficienttransposon-insertion mutations. These were detected as colonies that failedto stain with aniline blue, a (1-->3)-beta-glucan specific dye. Onefragment carried a biosynthetic gene cluster (locus I) containing theputative curdlan synthase gene, crdS, and at least two other crd genes. Thesecond fragment may contain only a single crd gene (locus II).Determination of the DNA sequence adjacent to several locus I mutationsrevealed homology to known sequences only in the cases of crdS mutations.Complete sequencing of the 1623 bp crdS gene revealed highest similaritiesbetween the predicted CrdS protein (540 amino acids) and glycosyltransferases with repetitive action patterns. These include bacterialcellulose synthases (and their homologs), which form(1-->4)-beta-glucans. No similarity was detected with putative(1-->3)- beta-glucan synthases from yeasts and filamentous fungi.Whatever the determinants of the linkage specificity of these beta-glucansynthases might be, these results raise the possibility that(1-->3)-beta-glucans and (1-->4)-beta-glucans are formed by relatedcatalytic polypeptides.