Construction of a chimeric gene cluster for the biosynthesis of apoemulsan with altered molecular weight

Diversity in the major polysaccharide antigen of Acinetobacter baumannii assessed by DNA sequencing, and development of a molecular serotyping scheme

We have sequenced the gene clusters for type strains of the Acinetobacter baumannii serotyping scheme developed in the 1990s, and used the sequences to better understand diversity in surface polysaccharides of the genus. We obtained genome sequences for 27 available serovar type strains, and identified 25 polysaccharide gene cluster sequences. There are structures for 12 of these polysaccharides, and in general the genes present are appropriate to the structure where known. This greatly facilitates interpretation. We also find 53 different glycosyltransferase genes, and for 7 strains can provisionally allocate specific genes to all linkages. We identified primers that will distinguish the 25 sequence forms by PCR or microarray, or alternatively the genes can be used to determine serotype by “molecular serology”. We applied the latter to 190 Acinetobacter genome-derived gene-clusters, and found 76 that have one of the 25 gene-cluster forms. We also found novel gene clusters and added 52 new gene-cluster sequence forms with different wzy genes and different gene contents. Altogether, the strains that have one of the original 25 sequence forms include 98 A. baumannii (24 from our strains) and 5 A. nosocomialis (3 from our strains), whereas 32 genomes from 12 species other than A. baumannii or A. nosocomialis, all have new sequence forms. One of the 25 serovar type sequences is found to be in European clone I (EC I), 2 are in EC II but none in EC III. The public genome strains add an additional 52 new sequence forms, and also bring the number found in EC I to 5, in EC II to 9 and in EC III to 2.

Pivotal roles of the outer membrane polysaccharide export and polysaccharide copolymerase protein families in export of extracellular polysaccharides in gram-negative bacteria

Many bacteria export extracellular polysaccharides (EPS) and capsular polysaccharides (CPS). These polymers exhibit remarkably diverse structures and play important roles in the biology of free-living, commensal, and pathogenic bacteria. EPS and CPS production represents a major challenge because these high-molecular-weight hydrophilic polymers must be assembled and exported in a process spanning the envelope, without compromising the essential barrier properties of the envelope. Emerging evidence points to the existence of molecular scaffolds that perform these critical polymer-trafficking functions. Two major pathways with different polymer biosynthesis strategies are involved in the assembly of most EPS/CPS: the Wzy-dependent and ATP-binding cassette (ABC) transporter-dependent pathways. They converge in an outer membrane export step mediated by a member of the outer membrane auxiliary (OMA) protein family. OMA proteins form outer membrane efflux channels for the polymers, and here we propose the revised name outer membrane polysaccharide export (OPX) proteins. Proteins in the polysaccharide copolymerase (PCP) family have been implicated in several aspects of polymer biogenesis, but there is unequivocal evidence for some systems that PCP and OPX proteins interact to form a trans-envelope scaffold for polymer export. Understanding of the precise functions of the OPX and PCP proteins has been advanced by recent findings from biochemistry and structural biology approaches and by parallel studies of other macromolecular trafficking events. Phylogenetic analyses reported here also contribute important new insight into the distribution, structural relationships, and function of the OPX and PCP proteins. This review is intended as an update on progress in this important area of microbial cell biology.