LINK TO ORIGINAL ARTICLE CORRESPONDENCE LINK TO Author’S REPLY glucose. It has been established that the level of cAMP in lactose-grown cells is low Is there any role for cAMP–CRP compared with other less-preferred carbon sources, but nevertheless is slightly higher in carbon catabolite repression of than the level of cAMP in glucose-grown cells9. Indeed, phosphorylated enzyme the Escherichia coli lac operon? IIAglc (which activates adenylate cyclase) is detected in lactose-grown cells but not Martine Crasnier-Mednansky in glucose-grown cells, as supported by the phosphorylation state of enzyme IIAglc In their review on carbon catabolite repres- case, then adding cAMP should have only detected by Western blotting10. sion (CCR) in bacteria, Boris Görke and a limited effect on diauxie or at least should Finally, other data suggest a role for Jörg Stülke (Carbon catabolite repression stimulate inducer exclusion by enhancing cAMP–CRP in diauxie. For example, a in bacteria: many ways to make the most glucose transport. Consequently, a further PTS sugar-like mannitol can substitute for out of nutrients. Nature Rev. Microbiol. 6, decrease in β-galactosidase expression in the glucose in diauxie production in E. coli 613–624 (2008))1 specifically analysed CCR presence of cAMP would result in enhance- whereas others, such as fructose, do not2. in Escherichia coli. The authors reported their ment of the diauxic lag. In any case, adding This correlates with the observation that view that inducer exclusion, by blocking cAMP would eliminate the diauxic lag. the level of cAMP in mannitol-grown cells the entry of lactose, is the main contributor The next contentious issue arises from the is in the same range as in glucose-grown to CCR when wild-type E. coli strains are observation that an E. coli mutant strain that cells, although the levels of cAMP in both growing in the presence of both glucose and is not sensitive to cAMP (lacL8UV5) exhibits mannitol-grown and glucose-grown cells are lactose. Under such growth conditions, E. coli diauxic growth in the presence of glucose lower than in fructose-grown cells11. strains generally exhibit diauxic growth, and lactose4. Based on this observation, Owing to the factors discussed above, which reflects the preferential use of glucose one can conclude, as reported4, that the low transcriptional regulation cannot be ruled over lactose and the effect of glucose in level of cAMP in cells growing on glucose out as a mechanism that supports diauxic preventing the use of lactose. The well estab- does not contribute to the manifestation growth in E. coli. However, it should be lished ‘classical’ interpretation of the glucose– of diauxie. However, it must be recollected emphasized that inducer exclusion (which lactose diauxie is that both inducer exclusion that the lacL8UV5 strain of E. coli growing has received less attention than transcrip- and the transcriptional positive regulation on lactose is impaired in β-galactosidase tional regulation) is also a major factor in of the lac operon by the cyclic AMP–CRP synthesis, and has a production rate of 60% diauxie and CCR in general. Undoubtedly, complex (cAMP–CRP) are necessary for full of a wild-type strain7. This is the most likely bacteria have developed multiple mecha- manifestation of diauxie, as first defined by explanation, besides inducer exclusion, nisms to assure the preferential use of a Jacques Monod2: two distinct exponential for the diauxic growth of strain lacL8UV5. carbon source, thereby allowing fast growth. growth phases are separated by a complete Indeed, the time necessary for synthesizing a Martine Crasnier-Mednansky is at the cessation of growth (lag phase). sufficient amount of β-galactosidase (for lac- Mednansky Institute, PO Box 940, Pine Valley, The contention that inducer exclusion tose utilization) will undoubtedly be longer California 91962, USA. is the main factor in diauxic growth is than with a wild-type strain. It thus cannot e-mail: [email protected] 3,4 1. Görke, B. & Stülke, J. Carbon catabolite repression in supported, in particular, by two articles be inferred that cAMP in a wild-type strain bacteria: many ways to make the most out of nutrients. that oppose the role of cAMP–CRP in the is irrelevant to diauxie, because a cAMP- Nature Rev. Microbiol. 6, 613–624 (2008). 2 Monod, J. Recherches sur la Croissance des Cultures transcriptional regulation of the lac operon insensitive mutant strain that is impaired Bactériennes. Thesis, Hermann et Cie, Paris (1942). in the presence of glucose. Prior to this in its ability to synthesize β-galactosidase 3. Inada, T., Kimata, K. & Aiba, H. Mechanism responsible for glucose–lactose diauxie in Escherichia coli: challenge view, the role of cAMP–CRP in diauxie was exhibits diauxie. Similarly, it should not be to the cAMP model. Genes Cells 1, 293–301 (1996). deduced based on data which indicated that concluded, as it has been before3, that expres- 4. Kimata, K., Takahashi, H., Inada, T., Postma, P. & Aiba, H. cAMP receptor protein-cAMP plays a crucial constitutive β-galactosidase synthesis (which sion of β-galactosidase is reduced by glucose role in glucose–lactose diauxie by activating the major does not require the inducer for synthesis) solely owing to inducer exclusion in a specific glucose transporter gene in Escherichia coli. Proc. Natl Acad. Sci. USA 94, 12914–12919 (1997). was repressed by glucose in the absence of mutant strain that lacks adenylate cyclase and 5. Ullmann, A. & Monod, J. Cyclic AMP as an antagonist of cAMP5 and that the rate of β-galactosidase possesses a cAMP-insensitive CRP (crp*). catabolite repression in Escherichia coli. FEBS Lett. 2, 57–60 (1968). synthesis in fully induced cells growing on Indeed, in such a mutant strain, expression of 6. Pastan, I. & Perlman, R. Cyclic adenosine monophosphate glucose was less than in cells growing on CRP* (encoded by crp*) is downregulated in in bacteria. Science 169, 339–344 (1970). 6 7. Silverstone, A. E., Arditti, R. R. & Magasanik, B. less-preferred carbon sources . Both obser- the presence of glucose owing to an auto-reg- Catabolite-insensitive revertants of lac promoter mutants. vations indicate that glucose transport by ulatory circuit that affects the transcription Proc. Natl Acad. Sci. USA 66, 773–779 (1970). 8. (REF. 8) Tagami, H., Inada, T., Kunimura, T. & Aiba, H. Glucose the phosphotransferase system (PTS) affect of crp* . Consequently, in the presence lowers CRP* levels resulting in repression of the lac β-galactosidase synthesis by reducing the of glucose, the amount of CRP* is reduced operon in cells lacking cAMP. Mol. Microbiol. 17, 251–258 (1995). level of cAMP. In addition, a typical diauxic compared with the amount of CRP in a 9. Crasnier, M. & Danchin, A. Characterization of lag was eliminated by adding exogenous wild-type strain. Therefore, besides inducer Escherichia coli adenylate cyclase mutants with modified 5 regulation. J. Gen. Microbiol. 136, 1825–1831 (1990). cAMP to the growth medium . exclusion, expression of β-galactosidase is 10. Hogema, B. M. et al. Inducer exclusion in Escherichia In support of the view that inducer exclu- further reduced by a limited amount of CRP coli by non-PTS substrates: the role of the PEP to pyruvate ratio in determining the phosphorylation state sion is the main factor in diauxie, cAMP– (CRP*), which in diauxie affects the transi- of enzyme IIAGlc. Mol. Microbiol. 30, 487–498 (1998). CRP was reported to play a crucial part in tion between growth phases. 11. Epstein, W., Rothman-Denes, L. B. & Hesse, J. Adenosine 3′:5′-cyclic monophosphate as mediator of diauxie only by activating the transcription of Another concern is the amount of cAMP catabolite repression in Escherichia coli. Proc. Natl the glucose transporter gene4. If this were the in cells growing on lactose compared with Acad. Sci. USA 72, 2300–2304 (1975). NATURE REVIEWS | MICROBIOLOGY www.nature.com/reviews/micro.