The Leucine\Lrp Regulon

The leucine/Lrp regulon is a recently described global response governed by a transcriptional regulator called the leucine-responsive regulatory protein (Lrp), a small basic protein (pI 9.2) composed of two identical subunits of molecular weight 18 800 daltons.1 Its sequence resembles only one other E. coli protein, AsnC, a positive regulator of asnA (the structural gene of asparagine synthetase A), with which it has 25% amino acid similarity. Cells deficient in Lrp show altered transcription of between 35 and 75 genes,23 increasing the expression of some and decreasing that of others.4–7 At some target promoters the presence of L-leucine in the growth medium modifies Lrp action, as observed with classic regulators, whereas at other LRP-regulated promoters leucine has little or no effect. Several reviews of the regulon have recently appeared.4,6,8,9