Genome-scale analysis of genetic regulatory elements in Streptomyces avermitilis MA-4680 using transcript boundary information

Background: The gram-positive bacterium, Streptomyces avermitilis, holds industrial importance as the producer of avermectin, a widely used anthelmintic agent, and a heterologous expression host of secondary metabolite-biosynthetic gene clusters. Despite its industrial importance, S. avermitilis' genome organization and regulation of gene expression remain poorly understood. In this study, four different types of Next-Generation Sequencing techniques, including dRNA-Seq, Term-Seq, RNA-Seq and ribosome profiling, were applied to S. avermitilis to determine transcription units of S. avermitilis at a genome-wide level and elucidate regulatory elements for transcriptional and translational control of individual transcription units.

Result: By applying dRNA-Seq and Term-Seq to S. avermitilis MA-4680, a total of 2361 transcription start sites and 2017 transcript 3'-end positions were identified, respectively, leading to determination of 1601 transcription units encoded in S. avermitilis' genome. Cataloguing the transcription units and integrated analysis of multiple high-throughput data types revealed the presence of diverse regulatory elements for gene expression, such as promoters, 5'-UTRs, terminators, 3'-UTRs and riboswitches. The conserved promoter motifs were identified from 2361 transcription start sites as 5'-TANNNT and 5'-BTGACN for the - 10 and - 35 elements, respectively. The - 35 element and spacer lengths between - 10 and - 35 elements were critical for transcriptional regulation of functionally distinct genes, suggesting the involvement of unique sigma factors. In addition, regulatory sequences recognized by antibiotic regulatory proteins were identified from the transcription start site information. Analysis of the 3'-end of RNA transcript revealed that stem structure formation is a major determinant for transcription termination of most transcription units.

Conclusions: The transcription unit architecture elucidated from the transcripts' boundary information provides insights for unique genetic regulatory mechanisms of S. avermitilis. Our findings will elevate S. avermitilis' potential as a production host for a diverse set of secondary metabolites.