Bm. Hogema et al., Autoregulation of lactose uptake through the LacY permease by enzyme IIA(Glc) of the PTS in Escherichia coli K-12, MOL MICROB, 31(6), 1999, pp. 1825-1833
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.