On 2017 Jul 11, Martine Crasnier-Mednansky commented:

The cya crp* mutant strain CA-8404 isolated by L. Soll (Sabourin D, 1975), which has been widely used for transduction of its crp* gene (also used in the present work), was finally characterized by Karimova G, 2004 as containing three mutations in the crp gene. Karimova G, 2004 further reported this CRP* was indeed capable of responding to cAMP and therefore was still sensitive to Carbon Catabolite Repression (CCR). Of interest to the present study, and any other studies aimed at releasing CCR in Escherichia coli, Karimova G, 2004 also characterized a novel CRP* with two mutations which totally relieved CCR as compared to the three-mutation CRP*.

Here cAMP-dependent CCR may not be the main culprit for preventing xylose utilization by the Escherichia coli wild type W strain (the Waksman’s strain). Generally, an increase in cAMP upon glucose depletion allows utilization of less-preferred sugar like xylose. Figure S7-A indicates that upon glucose exhaustion, E. coli W was still unable to use xylose after 96 hours, even though it could use xylose quite efficiently in the absence of glucose (Figure 2-D). In addition, the CRP* isolated by the authors (G141D) for specifically increasing xylose catabolism (Figure 1), and which doubled xylose utilization in the parent strain XW043, did not improve E. coli W xylose consumption at all in the presence of glucose (Figure 4-A). Thus, based on current knowledge, the inability of the Waksman’s strain to use xylose in the presence of a large excess glucose does not appear to relate to cAMP-dependent CCR. Interestingly, E. coli B, unlike W, is unable to use glucose fully when grown in excess glucose, and the typical increase in cAMP does not occur after cessation of growth (figure 2 in Peterkofsky A, 1971). Thus, if some glucose remains unused in the medium, cells may fail to use xylose.

On 2017 Jul 11, Martine Crasnier-Mednansky commented:

The cya crp* mutant strain CA-8404 isolated by L. Soll (Sabourin D, 1975), which has been widely used for transduction of its crp* gene (also used in the present work), was finally characterized by Karimova G, 2004 as containing three mutations in the crp gene. Karimova G, 2004 further reported this CRP* was indeed capable of responding to cAMP and therefore was still sensitive to Carbon Catabolite Repression (CCR). Of interest to the present study, and any other studies aimed at releasing CCR in Escherichia coli, Karimova G, 2004 also characterized a novel CRP* with two mutations which totally relieved CCR as compared to the three-mutation CRP*.

Here cAMP-dependent CCR may not be the main culprit for preventing xylose utilization by the Escherichia coli wild type W strain (the Waksman’s strain). Generally, an increase in cAMP upon glucose depletion allows utilization of less-preferred sugar like xylose. Figure S7-A indicates that upon glucose exhaustion, E. coli W was still unable to use xylose after 96 hours, even though it could use xylose quite efficiently in the absence of glucose (Figure 2-D). In addition, the CRP* isolated by the authors (G141D) for specifically increasing xylose catabolism (Figure 1), and which doubled xylose utilization in the parent strain XW043, did not improve E. coli W xylose consumption at all in the presence of glucose (Figure 4-A). Thus, based on current knowledge, the inability of the Waksman’s strain to use xylose in the presence of a large excess glucose does not appear to relate to cAMP-dependent CCR. Interestingly, E. coli B, unlike W, is unable to use glucose fully when grown in excess glucose, and the typical increase in cAMP does not occur after cessation of growth (figure 2 in Peterkofsky A, 1971). Thus, if some glucose remains unused in the medium, cells may fail to use xylose.