Short Views on Insect Genomics and Proteomics by Chandrasekar Raman Marian R. Goldsmith & Tolulope A. Agunbiade

Author:Chandrasekar Raman, Marian R. Goldsmith & Tolulope A. Agunbiade
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.2.3 Differences in Prokaryotic Sec Machinery

The process of Sec biosynthesis is essentially identical in eukaryotes and archaea, with archaeal proteins homologous to eukaryotic SPS, PSTK, and SecS [54]. In contrast, bacteria have no homologue to PSTK; the serine charged on tRNAsec is transformed to selenocysteine without prior activation by phosphorylation. The bacterial selenocysteine synthase (called SelA) has very different sequence and structure than its eukaryotic/archaeal counterpart [55], casting doubt on their common descent.

The process of Sec insertion (translational recoding) presents stronger differences across the three domains of life. In archaea, SECIS elements are found in the 3’UTR, as in eukaryotes (although there is one documented case where, surprisingly, it is located in the 5’UTR [56]). However, their sequence and structure have very poor similarity with eukaryotic SECIS elements, primarily for the absence of a kink-turn motif and conserved quartet. To date, despite all efforts, no SECIS binding protein has been identified in archaea, leaving open the question of how the SECIS and the Sec site communicate [57].

Bacterial SECIS elements reside within the coding sequence, next to (or including) the Sec-UGA [58]. Their structure does not resemble either archaeal or eukaryotic SECIS elements [40]. Bacterial EFsec (called SelB) recognizes SECIS elements directly through its C-terminal domain, whereas its N-terminal domain works, again, as an elongation factor. Despite these differences, the main core of the Sec machinery is similar enough in the three domains to suggest a common descent (tRNAsec, SelB/EFsec, SelD/SPS). For this reason, it is generally believed that Sec evolved just once during evolution.

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