The Rhizobium nodulation genes nodABC are involved in the synthesis of lipo-chitin oligosaccharides. We have analysed the metabolites which are produced in vivo and in vitro by Rhizobium strains which express the single nodA, nodB and nodC genes or combinations of the three. In vivo radioactive labelling experiments, in which D-[1-14C]-glucosamine was used as a precursor, followed by mass spectrometric analysis of the purified radiolabelled metabolic products, showed that Rhizobium strains that only express the combination of the nodB and nodC genes do not produce lipo-chitin oligosaccharides but instead produce chitin oligomers (mainly pentamers) which are devoid of the N-acetyl group on the non-reducing terminal sugar residue (designated NodBC metabolites). Using the same procedure we have shown that when the nodL gene is expressed in addition to the nodBC genes the majority of metabolites contain an additional O-acetyl substituent on the non-reducing terminal sugar residue (designated NodBCL metabolites). The NodBC and NodBCL metabolites purified after in vivo labelling were compared with the radiolabelled metabolites produced in vitro by Rhizobium bacterial cell lysates to which UDP-N-acetyl-D-[U-14C]-glucosamine was added using thin-layer chromatography. The results show that the lysates of strains which expressed the nodBC or nodBCL genes can also produce NodBC and NodBCL metabolites. The same results were obtained when the NodB and NodC proteins were produced separately in two different strains. On the basis of these and other recent results, we propose that NodB is a chitin oligosaccharide deacetylase, NodC an N-acetylglucosaminyltransferase and, by default, NodA is involved in lipid attachment.

All lipo-chitin oligosaccharides identified from Rhizobium leguminosarum carry an O-acetyl moiety on C6 of the nonreducing terminal N-acetylglucosamine residue. Previously, we have shown that purified NodL protein, using acetyl-CoA as acetyl donor, in vitro acetylates N-acetylglucosamine, chitin oligosaccharides, and lipo-chitin oligosaccharides. In this paper, the enzymatic properties and substrate specificity of NodL protein were analyzed, using a spectrophotometric assay to quantify NodL transacetylating activity. NodL functions optimally under alkaline conditions. Transacetylating activity has a broad temperature optimum between 28 and 42 degrees C. NodL protein is stable for at least 15 min up to 48 degrees C. Glucosamine, chitosan oligosaccharides, terminally de-N-acetylated chitin derivatives, and cellopentaose were identified as acetyl-accepting substrates for NodL protein. Quantitative substrate specificity studies show that chitin derivatives with a free amino group on the nonreducing terminal residue are the preferred substrates of the NodL protein. Our results strongly indicate that the nonreducing terminally de-N-acetylated chitin oligosaccharides produced by the NodC and NodB enzymes are the in vivo acetyl-accepting substrates for NodL protein.

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How head patterning is regulated in vertebrates is yet to be understood. In this study, we show that frog embryos injected with Noggin at different blastula and gastrula stages had their head development sequentially arrested at different positions. When timed BMP inhibition was applied to BMP-overexpressing embryos, the expression of five genes: xcg-1 (a marker of the cement gland, which is the front-most structure in the frog embryo), six3 (a forebrain marker), otx2 (a forebrain and mid-brain marker), gbx2 (an anterior hindbrain marker) and hoxd1 (a posterior hindbrain marker) were sequentially fixed. These results suggest that timed interactions between BMP and anti-BMP are involved in patterning the vertebrate head progressively in time and space. Since the above genes are not expressed sequentially, there may be a BMP dependent gene sequence during head patterning that can be arrested by BMP inhibition and regulate the specification of positional values in the head.