Moonlighting glutamate formiminotransferases can functionally replace 5-formyltetrahydrofolate cycloligase.

5-Formyltetrahydrofolate (5-CHO-THF) is formed by a side reaction of serine hydroxymethyltransferase. Unlike other folates, it is not a one-carbon donor but a potent inhibitor of folate enzymes and must therefore be metabolized. Only 5-CHO-THF cycloligase (5-FCL) is generally considered to do this. However, comparative genomic analysis indicated (i) that certain prokaryotes lack 5-FCL, implying that they have an alternative 5-CHO-THF-metabolizing enzyme, and (ii) that the histidine breakdown enzyme glutamate formiminotransferase (FT) might moonlight in this role. A functional complementation assay for 5-CHO-THF metabolism was developed in Escherichia coli, based on deleting the gene encoding 5-FCL (ygfA). The deletion mutant accumulated 5-CHO-THF and, with glycine as sole nitrogen source, showed a growth defect; both phenotypes were complemented by bacterial or archaeal genes encoding FT. Furthermore, utilization of supplied 5-CHO-THF by Streptococcus pyogenes was shown to require expression of the native FT. Recombinant bacterial and archaeal FTs catalyzed formyl transfer from 5-CHO-THF to glutamate, with k(cat) values of 0.1-1.2 min(-1) and K(m) values for 5-CHO-THF and glutamate of 0.4-5 µM and 0.03-1 mM, respectively. Although the formyltransferase activities of these proteins were far lower than their formiminotransferase activities, the K(m) values for both substrates relative to their intracellular levels in prokaryotes are consistent with significant in vivo flux through the formyltransferase reaction. Collectively, these data indicate that FTs functionally replace 5-FCL in certain prokaryotes.

Evidence of physiological phototropin1 (phot1) action in response to UV-C illumination.

Stem growth kinetics were measured in cucumber (Cucumis sativus) and Arabidopsis thaliana using highly-sensitive monitoring with 5-minute resolution, in darkness and in response to a short, single pulse of UV-C illumination. The results show that UV-C, like blue light, induces a rapid decrease in seedling growth rate. The fluence-response kinetics and time course were similar to the phototropin1 mediated response observed following a blue pulse. Arabidopsis seedlings were used to assess the genetic mechanism of this response. The phot1 mutant exhibited defects in stem growth rate inhibition, with sustained growth inhibition completely absent following specific treatments. The cryptochrome and phytochrome mutants exhibited responses comparable to wild type, suggesting that these receptor classes do not contribute to this response. The work demonstrates in two species that UV-C has an effect on a rapid plant photomorphogenic response and that the response is partially mediated by the phot1 photoreceptor.