Growth Rate Regulation of lac Operon Expression in Escherichia coli

Growth Rate Regulation of lac Operon Expression in Escherichia coli

Citation Information

  • Journal: FEBS Letters 553

  • Pages: 397-402

  • Year: 2003

  • Authors: Jong-Tar Kuo, Yu-Jen Chang, Ching-Ping Tseng

  • Institution: Department of Biological Science and Technology, National Chiao Tung University, Taiwan R.O.C.

  • Received: 30 July 2003; Revised: 4 September 2003; Accepted: 9 September 2003

  • Published Online: 26 September 2003

  • Editor: Gianni Cesareni

Abstract

  • Key Findings:

    • The transcription of the lac operon is regulated inversely with cell growth rate, contrasting with ribosomal RNA gene expression that increases with growth rate.

    • Growth rate impact on lac promoter is independent of carbon substrates and chromosomal location.

    • Key regulator: Cyclic adenosine monophosphate (cAMP) which decreases with increased cell growth rate.

    • Analyzed plac-lacZ reporter fusion in cya mutant across various growth conditions.

    • Results showed that plac-lacZ expression in the cya mutant was lower and not dependent on growth rate.

    • Findings indicate that cAMP mediates the growth rate-dependent regulation of lac operon expression in Escherichia coli.

Keywords

  • Growth rate

  • Lac operon

  • Cyclic adenosine monophosphate (cAMP)

  • Escherichia coli

1. Introduction

  • The transcription level of most genes in Escherichia coli varies with nutrient growth conditions due to operon-specific activators or repressors.

    • Higher nutrient availability results in shorter doubling times.

    • Regulation of over 100 individual proteins in E. coli demonstrates a linear correlation with growth rate.

  • Growth Rate-Dependent Regulation: Refers to the coordination of gene expression with cellular growth rate.

    • Effects of Growth Rate on Gene Expression:

    • tRNA Synthesis: Growth rate influences tRNA synthesis.

    • rRNA and Ribosomal Protein Synthesis: These decrease as growth rate decreases, indicating a negative feedback regulation.

    • Notably articulated genes include:

    • Fumarate Reductase (frdABCD)

    • Isocitrate Dehydrogenase (icd)

    • Succinate Dehydrogenase (sdhCDAB)

    • ATPase (atpIBEFHAGDC)

    • These genes display increased expression at lower growth rates.

Role of Cyclic Adenosine Monophosphate (cAMP)

  • cAMP plays a crucial role in the induction of catabolic enzymes in E. coli through the glucose effect.

  • cAMP Receptor Protein (CRP):

    • Binds to cAMP, facilitating transcriptional activation or repression of various promoters.

  • Intracellular cAMP levels vary based on nutrient composition, with earlier studies indicating that dilution rate changes typically do not affect cAMP levels noticeably.

    • Difficulties in measuring low cellular cAMP levels have been noted.

  • Recent findings by Notley et al. establish that low cell growth rates in glucose-limited chemostats see elevated cAMP concentrations.

    • Understanding cAMP levels across various carbon substrates and growth rates is vital for comprehending gene regulation by cAMP in E. coli.

The Lac Operon

  • The lac operon is foundational for studying molecular biology of gene expression and regulation.

  • Expression is regulated by two key molecules:

    • Lac Repressor: Blocks transcription in the absence of lactose.

    • CRP: Activates transcription in the presence of cAMP.

  • Transcription Repression by Glucose:

    • Glucose suppresses lac operon transcription by lowering intracellular cAMP concentrations.

    • In glucose absence, cAMP accumulates and binds to CRP to enhance transcription by forming the cAMP-CRP complex which attaches to the CRP binding region near the lac promoter.

  • Catabolite Repression Model:

    • Supported by evidence that glucose reduces crp gene expression, thus lowering CRP levels within the cell.

  • Physiological regulation of the lac operon shows minimal changes in β-galactosidase activity under varied carbon substrates and growth rates.

    • Many catabolite-insensitive lac promoters demonstrate diminished expression primarily at high growth rates in rich media.

Objective of the Study

  • The study aims to explore the mechanisms behind the growth rate regulation of the wild-type lac promoter utilizing continuous culture to control growth rates while maintaining substrate type.

    • Methodology: Investigated how cell growth rate affected plac-lacZ reporter fusion expression across different carbon substrates.

    • Findings: Established that growth rate regulates plac-lacZ expression, and this regulation is contingent on the presence of cAMP in E. coli.

Conclusion

  • This study elucidates the pivotal role of cAMP in mediating the growth rate-dependent regulation of the lac operon in Escherichia coli as evidenced by the experiments conducted with the cya mutant.