You are studying how a certain species of grass decomposes in a semi-arid grassl

Question

You are studying how a certain species of grass decomposes in a semi-arid grassland in southern Arizona to get an idea on the C dynamics of the region. You find that after a few months the grass litter you’ve been sampling has a higher abundance of fungi than earlier samplings coupled with a decreased abundance of bacteria. You compare the chemical composition of the earlier litter samplings with the older litter and you find that this species of grass has a high concentration of cellulose and the cell solubles (sugars like glucose) have declined rapidly over the experiment.

A. You know that cellulase expression in microbial decomposers is contingent on Carbon Catabolite Repressor (CCR) which is a cellulose sensor and works the same as the lac repressor in E. coli. Would CCR be bound to the operator on the older litter? Please explain why in 1-3 sentences.

B. You find a healthy mixture of bacteria and fungi growing on the earlier litter samplings but you don’t find any bacteria on the older litter. What could explain these findings? Explain in terms of cellulose and cellulose monomers.

Explanation / Answer

A.  Carbon catabolite repression (CCR) is a phenomenon in microorganisms in which the presence of a carbon source in the medium can repress expression of certain genes and operons, whose gene products are often concerned with the utilization of alternative carbon sources. In bacteria glucose is the most readily utilized carbon sourse as food. In the presence of glucose the operons for other carbohydrates are switched off by regulatory mechanisms.

The earlier litter was rich in bacterial species and the glucose levels were higher as compared to the older litter. In the older litter there is lower level of glucose because the bacteria utilized glucose as energy source as it is most preferred. Yes, CCR would be bound to the operator in older litter. Until glucose is completely utilized no other carbohydrate source can be used. CCR will repressor the function of genes involved in the metabolism of other sources of energy such as cellulose.

B. The glucose is consumed by bacteria and the lower levels of glucose inhibits the growth of bacteria. As the cellulose levels are higher fungi can grow easily. Most food that a fungus might consume is composed of complex, organic compounds, e.g., cellulose, lignin, pectin, starch, etc., which is insoluble. In order for this food to be utilized by the fungus, it must be broken down into simpler molecules that can be transported through their cell walls. Fungi produce an enzyme that breaks down cellulose and helps them derive energy.

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