Autoregulation of lactose uptake through the LacY permease by enzyme IIA(Glc) of the PTS in Escherichia coli K-12

Bacterial growth on one or more carbon sources requires careful control of the uptake and metabolism of these carbon sources. In Escherichia coli, the phosphorylation state of enzyme IIA(Glc) of the phosphoenolpyruvate:carboh ydrate phosphotransferase system (PTS) is involved in this control in two w ays. The un phosphorylated form of IIA(Glc) causes 'inducer exclusion', the inhibition of uptake of a number of non-PTS carbon sources, including lact ose uptake by the lactose permease, The phosphorylated form of enzyme IIA(G lc) probably activates adenylate cyclase, In cells growing on lactose, enzy me IIA(Glc) was approximately 50% dephosphorylated, suggesting that lactose could inhibit its own uptake. This inhibition could be demonstrated by com paring lactose uptake rates in the wildtype strain and in a mutant in which the lactose carrier was insensitive to inducer exclusion. In this deregula ted mutant strain, lactose was consumed much faster, and large amounts of g lucose were excreted. It was shown that enzyme IIA(Glc) was dephosphorylate d more strongly and that the cAMP level was lower in the mutant, most proba bly causing the observed decrease in lac expression level. When the lac exp ression level in the mutant strain was increased to that of the parent stra in by adding exogenous cAMP, growth on lactose was slower, suggesting that enzyme IIA(Glc) mediated inhibition of lactose uptake and downregulation of the lac expression level protected the cells against excessive lactose inf lux. An even stronger increase in the lac expression level in a mutant lack ing enzyme IIA(Glc) caused complete growth arrest. We conclude that the aut oregulatory mechanism that controls lactose uptake is an important mechanis m for the cells in adjusting the uptake rate to their metabolic capacity.