What are the similarities and differences between obligate pathogens and free-living saprotrophs in the ways they control the rate and type of biosynthesis?

Question

What will be an ideal response?

Answer 1

Each species makes an evolutionary "choice" whether to maintain the expense of a particular assimilatory or biosynthetic pathway, or to lose the pathway and become dependent on other species in the environment. Parasitic microbes that grow only inside a host cell show extensive loss of biosynthetic genes. Since energy is required to run this biosynthetic machinery, as well as maintain and replicate the genome, parasites that can rely on their hosts for biosynthesis and energy have lost many abilities to generate energy themselves (or conserve it in molecules like ATP and NADH). This is referred to as genome degeneration.

Free-living bacteria, by contrast, often divert carbon resources away from growth to produce secondary products, which are not essential nutrients, but can enhance nutrient uptake in low-nutrient environments or inhibit competitors. The free-living bacteria that run the Calvin cycle also control rates of biosynthesis by concentrating CO2 near Rubisco using carboxysomes.

However, both pathogens (such as Legionella pneumophila) and free-living bacteria (such as certain E. coli strains) regulate certain biosynthesis pathways in similar ways. For example, because fatty acid synthesis is expensive and needs to be balanced with growth of the cell, it is regulated by starvation in both species. Fatty acid synthesis can be regulated by other mechanisms as well:
1) Acetyl-CoA carboxylase regulates its own transcription (similar to catabolite repression). Transcription of an operon encoding two subunits of acetyl-CoA carboxylase (AccB, AccC) is repressed by one of its subunits, protein AccB. As AccB increases in concentration, it binds the promoter of the accBC operon, repressing further transcription.
2) In E. coli, low temperature favors unsaturated fatty acids because they are less rigid and maintain membrane flexibility. Cold will initiate the dehydratase enzyme that desaturates the fatty acid bond.