The cost of expression of Escherichia coli lac operon proteins is in the process, not in the products

Abstract

Transcriptional regulatory networks allow bacteria to express proteins only when they are needed. Adaptive hypotheses explaining the evolution of regulatory networks assume that unneeded expression is costly and therefore decreases fitness, but the proximate cause of this cost is not clear. We show that the cost in fitness to Escherichia coli strains constitutively expressing the lactose operon when lactose is absent is associated with the process of making the lac gene products, i.e., associated with the acts of transcription and/or translation. These results reject the hypotheses that regulation exists to prevent the waste of amino acids in useless protein or the detrimental activity of unnecessary proteins. While the cost of the process of protein expression occurs in all of the environments that we tested, the expression of the lactose permease could be costly or beneficial, depending on the environment. Our results identify the basis of a single selective pressure likely acting across the entire E. coli transcriptome.

Figure 1.—

Rate of α-PNPG translocation by the constitutive strain (DMS269, “+”), the constitutive ΔlacZ strain (DMS1346, circles), and a strain with at least five copies of the lac operon (DD2460, triangles). At low substrate concentrations, the activities of DMS269 and DMS1346 are identical, indicating that the amount of LacY is the same in the two strains. As a control, DD2460 has much higher rates of transport at low substrate concentrations, as would be expected if the availability of LacY were rate limiting for transport.

Figure 2.—

Competition experiments to test for the cost of amino acid usage and cost of production on glucose. A constitutive ssrA-tagged strain (DMS1397) competed against a constitutive strain (DMS269, open circles) and against a regulated strain (DMS267, solid circles). (B) The constitutive ΔRBS strain (DMS1360) competed against the constitutive strain (DMS269, open circles) and regulated strain (DMS267, solid circles). In A and B, the line is the selection difference between regulated and constitutive, 10.28%/generation. (A) The ssrA tag, which recycles protein, does not alleviate the cost of expression. (B) Abolishing the act of production alleviates all of the cost of expression.

Figure 3.—

Competition experiments to determine why constitutivity is more costly on glucose (open circles and dotted lines) than maltose (solid circles and solid lines). Competition of a regulated strain (DMS267) and a constitutive ΔlacY strain (DMS1333) on glucose and maltose shows that the cost of LacZ and LacA is the same on both carbon sources (6.39 and 6.09%/generation, respectively). (B) A regulated strain (DMS267) is more fit than a constitutive ΔlacZ strain (DMS1346) on glucose (3.43%/generation), but ΔlacZ is more fit than the same regulated strain on maltose (2.54%/generation), indicating that LacY is costly on glucose but beneficial on maltose.