Does DG42 synthesize hyaluronan or chitin?: A controversy about oligosaccharides in vertebrate development

摘要

Science generally progresses in slow but deliberate increments, which are punctuated by major advances in concept or fact. However, the latter are rare and not infrequently go unrecognized when they first occur. Another event that can add spice to a field, and attract the attention of scientists from outside the discipline, is a genuine controversy. One such controversy is presented by two reports in this issue of the Proceedings. To be asked to referee such a controversy is an interesting but difficult task, since both groups have significant data to back their claims. The story begins in 1983, when Igor Dawid and colleagues reported the isolation of several genes that are Differentially expressed at Gastrulation (DG) in embryos of the frog, Xenopus laevis. One of these, the endoderm-specific DG42, is expressed in a short window during embryogenesis, being first detected after the midblastula stage, peaking at late gastrula, and decaying by the end of neuralation. Appropriate probes were used to show that the messenger RNA and predicted protein product move in a wave or gradient through the embryo, with the last remnants seen in the ventral regions of the gut at the tailbud stage. For a while thereafter, DG42 remained an interesting gene in search of a function. As often happens, the first clues came from unexpected sequence homology information. When it was first cloned, DG42 showed no obvious homologies to any previously known protein or gene. Subsequently, some similarities were found with fungal chitin synthases and with the rhizobium NodC gene that is known to synthesize chitin oligomers.