Protein Biosynthesis - Purdue University
it isn't involved in the process by which the ..
The involvement of the two glycosyltransferase auxiliary proteins in spiramycin biosynthesis was also elucidated. Our results showed that both of them are involved in glycosylation. The two glycosyltransferases Srm5 and Srm29 require the assistance of an auxiliary protein to perform the attachment of mycaminose and forosamine, respectively. The genes encoding these glycosyltransferases are both immediately downstream of the genes encoding the auxiliary proteins. Each glycosyltransferase can be activated by the auxiliary protein encoded by the upstream gene: Srm5 by Srm6 and Srm29 by Srm28. In addition, we have demonstrated the existence of an interplay between the auxiliary proteins (i.e., Srm28 can also efficiently activate Srm5). This explains why srm6 is dispensable for spiramycin biosynthesis. In contrast, we did not observe any activation of Srm29 by Srm6. Because of the observed flexibility of auxiliary proteins, we cannot exclude that Srm38, the mycarosyltransferase, might require the assistance of an auxiliary protein, but this cannot be studied in vivo in S. ambofaciens.
Steps involved in protein synthesis are ..
There are three deoxysugar moieties in the spiramycin molecule: two amino sugars, mycaminose and forosamine, and one neutral sugar, mycarose (Fig. ). Enzymes involved in the biosynthesis of these sugars are encoded by genes in the spiramycin biosynthetic gene cluster (). The analysis of the cluster revealed the presence of four genes (srm5, srm29, srm30, and srm38) encoding putative glycosyltransferases () (Fig. ). This was unexpected as there is usually one specific glycosyltransferase for each sugar to be transferred. To explain the discrepancy between the number of putative glycosyltransferases and the number of sugar moieties, several hypotheses can be formulated. For instance, one of these genes might not be expressed. However, transcriptional analysis of the cluster showed that all four genes are transcribed during spiramycin biosynthesis, and their transcription is coregulated with that of the other biosynthetic genes (F. Karray et al., unpublished data). Alternatively, the product of one of the genes might be inactive, but comparisons and alignments with the protein sequences of functional glycosyltransferases did not provide any obvious indication that this might be the case. In that light, if all of the glycosyltransferases are expressed and active, two of them could independently catalyze the attachment of the same sugar. Finally, one of the four glycosyltransferases may be involved in a process other than spiramycin biosynthesis, for instance, in macrolide resistance by drug inactivation, which has been observed in Streptomyces antibioticus, a producer of the macrolide oleandomycin ().