Acyl phosphates represent a mixed anhydride class of compounds whose lability allows the phosphorylation of an aspartyl carboxyl contained in a protein to reversibly induce changes in structure that may have biological significance, particularly in prokaryotic systems. In this report the phosphorelay system that regulates sporulation inBacillus subtilis is described briefly and its analogy to other regulatory systems is outlined. The structural properties of the aspartate containing second component of the phosphorelay system SpoOF is examined by multidimensional NMR techniques and comparison is made with a known sequence analog, CheY. Distinct differences are apparent that are reflected by the extended half life of phosphorylated SpoOF relative to the CheY analog. It is probable that in a general way the distinct half life characteristics may be related to the differing functions of the various regulatory aspartate containing proteins in the cell.