Abstract
The general secretion (Sec) pathway plays a prominent role in bacterial protein export, and the accessory component SecDF has been shown to improve transportation efficiency. Inspection of Streptomyces coelicolor genome reveals the unexpected presence of two different forms of secDF homologous genes: one in fused form (secDF) and the other in separated form (secD and secF). However, the functional role of two SecDF homologs in S. coelicolor has not yet been determined. Transcriptional analysis of secDF homologs reveals that these genes are constitutively expressed. However, the transcript levels of secD and secF are much higher than that of secDF in S. coelicolor. Deletion of secDF or/and secD/secF in S. coelicolor did result in reduced secretion efficiency of Xylanase A and Amylase C, suggesting that they may have redundant functions for Sec-dependent translocation pathway. Moreover, our results also indicate that SecD/SecF plays a more prominent role than SecDF in protein translocation. Evolutionary analysis suggests that the fused and separated SecDF homologs in Streptomyces may have disparate evolutionary ancestries. SecD/SecF may be originated from vertical transmission of existing components from ancestor of Streptomyces species. However, SecDF may be derived from bacterial ancestors through horizontal gene transfer. Alternately, it is also plausible that SecDF may have arisen through additional gene duplication and fusion events. The acquisition of a second copy may confer a selective benefit to Streptomyces by enhancing protein transport capacity. Taken together, our results provide new insights into the potential biological function and evolutionary aspects of the prokaryotic SecDF complex.
Highlights
Streptomycetes are soil-dwelling Gram-positive bacteria that have a large (6.5–11.9 Mb) linear chromosome with high GC content, containing a central region that is highly conserved throughout the genus, and terminal regions that are variable in composition and organization [1,2]
Protein transport across the bacterial membrane is mediated by different translocation systems, of which the general protein secretion (Sec) system plays a prominent role in protein export and membrane insertion [11,12,13,14,15]
Our results here show that the simultaneous depletion of these genes results in severe inhibition of Xylanase A (XlnA) and Amylase C (AmlC) secretion, suggesting that SecDF and SecD-F may have redundant functions in the Sec protein translocation pathway
Summary
Streptomycetes are soil-dwelling Gram-positive bacteria that have a large (6.5–11.9 Mb) linear chromosome with high GC content, containing a central region that is highly conserved throughout the genus, and terminal regions that are variable in composition and organization [1,2]. Our results here show that the simultaneous depletion of these genes results in severe inhibition of Xylanase A (XlnA) and Amylase C (AmlC) secretion, suggesting that SecDF and SecD-F may have redundant functions in the Sec protein translocation pathway. Construction of S. coelicolor mutant strains The secD, secF and secDF genes were disrupted by in frame deletion via double-crossover homologous recombination [28] with the help of the temperature sensitive plasmid pKC1139 [29].
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