All organisms must synthesize the basic nucleotides which make up their genetic code. This synthesis is especially important in growing tissues. Two pathways exist for the synthesis of the pyrimidine bases: the “de novo” pathway, which makes them basically from scratch, beginning with bicarbonate, glutamine, and ATP, and the “salvage” pathway, which creates them from pyrimidine nucleosides (Minic et al. 2001). R. pachyptila does not possess the enzymes necessary to complete the “de novo” pathway on its own (Simon et al. 2000). The first three enzymes necessary for this synthesis are found only in the troposome tissue of the worm and are created exclusively by its symbiont (Minic et al. 2001).
Adapted from figure 2 (Minic et al. 2001) Troposomal extracts were placed on top of a chromatography column and fractions were assayed for enzyme activity.
R. pachyptila must gain all of its pyrimidines either through a direct transfer from the symbiont, or by digesting some of its symbionts and converting their nucleosides into its own nucleotides (Minic et al. 2001).
Both the worm and the symbiont possess the ending enzymes of the “de novo” sequence. But these enzymes have their maximum efficiencies at drastically different pHs. R. pachyptila’s enzyme functions best at a pH of 7.2, the symbionts functions best at a pH of 8.7. The symbiont’s enzymes can even operate at a pH of 9.6. this pH is higher than that which any other organism can tolerate for this enzyme. Between the two of them, their enzymes function across a uniquely broad spectrum, allowing them to cope with the extreme and variable environment in which they live (Minic et al. 2001).
A later study discovered that even within the troposome a complex structure regulates where the enzymes of the “de novo” pathway are most concentrated and thus most active (Hervé and Minic 2007).
Adapted from figure 2 (Hervé and Minic 2007). Variations of enzyme activities across the troposome
They function most in the center of the troposome where the synthesizing bacteria are most concentrated and most metabolically active. In contrast the digestive enzymes, which break down the nucleotides to convert them into R. pachyptila’s own nucleotides increase in concentration as you move toward the periphery (Hervé and Minic 2007).
Adapted from figure 2 (Hervé and Minic 2007). Variations of enzyme activities across the troposome
Lysosomes and partially degraded bacteria also increase as you move toward the periphery of the troposome, as electron microscopy reveals. Apparently R. pachyptila uses its symbionts as a direct source of nutrition in addition to exploiting their ability to fix carbon (Hervé and Minic 2007).