Among the few regulatory proteins encoded by Mycoplasma pneumoniae is HPr kinase/phosphatase (HPrK/P), the key regulator of carbon metabolism in low-GC Gram-positive bacteria. The corresponding gene, hprK, and the gene encoding the target protein HPr, ptsH, were overexpressed. In vitro analysis of the purified proteins confirmed ATP-dependent phosphorylation of HPr by HPrK/P. In contrast to HPrK/P of Bacillus subtilis, which is by default a phosphatase and needs high ATP concentrations for kinase activity, the M. pneumoniae enzyme exhibits kinase activity at very low ATP concentrations and depends on P i for phosphatase activity. This inverted control of enzymic activity may result from the adaptation to very different ecological niches. While the standard activities of HPrK/P from M. pneumoniae and other Grampositive bacteria differ, they are both modulated by the concentration of ATP, P i and glycolytic intermediates. Site-directed mutagenesis of a potential ATPbinding site and of the HPrK/P signature sequence resulted in four different activity classes : (i) inactive proteins, (ii) enzymes with reduced kinase and phosphatase activities, (iii) enzymes that had lost phosphatase, but not kinase activity, and (iv) enzymes that exhibited increased phosphatase activity.