BACKGROUND: The universal stress protein UspA is a small cytoplasmic bacterial protein whose expression is enhanced several-fold when cellular viability is challenged with heat shock, nutrient starvation, stress agents which arrest cell growth, or DNA-damaging agents. UspA enhances the rate of cell survival during prolonged exposure to such conditions, suggesting that it asserts a general "stress endurance" activity. However, neither the structure of UspA nor the biochemical mechanism by which it protects cells from the broad spectrum of stress agents is known. RESULTS: The crystal structure of Haemophilus influenzae UspA reveals an asymmetric dimer with a tertiary alpha/beta fold similar to that of the Methanococcus jannaschi MJ0577 protein, a protein whose crystal structure revealed a novel ATP binding motif. UspA differs significantly from the MJ0577 structure in several details, including the triphosphate binding loop of the ATP binding motif; UspA shows no ATP binding activity. CONCLUSIONS: Within the universal stress protein family that is delineated by sequence similarity, UspA is the only member which has been correlated with a cellular activity, and MJ0577 is the only member which has been assigned a biochemical activity, i.e., ATP binding. UspA has a similar fold to the MJ0577 protein but does not bind ATP. This suggests that members of this protein family will segregate into two groups, based on whether or not they bind ATP. By implication, one subset of the universal stress proteins presumably has an ATP-dependent function, while another subset functions in ATP-independent activities.