Escherichia coli mRNAs with strong Shine/Dalgarno sequences also contain 5' end sequences complementary to domain # 17 on the 16S ribosomal RNA

Sarecycline inhibits protein translation in Cutibacterium acnes 70S ribosome using a two-site mechanism

Acne vulgaris is a chronic disfiguring skin disease affecting ∼1 billion people worldwide, often having persistent negative effects on physical and mental health. The Gram-positive anaerobe, Cutibacterium acnes is implicated in acne pathogenesis and is, therefore, a main target for antibiotic-based acne therapy. We determined a 2.8-Å resolution structure of the 70S ribosome of Cutibacterium acnes by cryogenic electron microscopy and discovered that sarecycline, a narrow-spectrum antibiotic against Cutibacterium acnes, may inhibit two active sites of this bacterium's ribosome in contrast to the one site detected previously on the model ribosome of Thermus thermophilus. Apart from the canonical binding site at the mRNA decoding center, the second binding site for sarecycline exists at the nascent peptide exit tunnel, reminiscent of the macrolides class of antibiotics. The structure also revealed Cutibacterium acnes-specific features of the ribosomal RNA and proteins. Unlike the ribosome of the Gram-negative bacterium Escherichia coli, Cutibacterium acnes ribosome has two additional proteins, bS22 and bL37, which are also present in the ribosomes of Mycobacterium smegmatis and Mycobacterium tuberculosis. We show that bS22 and bL37 have antimicrobial properties and may be involved in maintaining the healthy homeostasis of the human skin microbiome.

Size-variable zone in V3 region of 16S rRNA

The size distribution of complete 16S-rRNA sequences from the SILVA-database and nucleotide shifts that might interfere with the secondary structure of the molecules were evaluated. Overall, 513,309 sequences recorded in SILVA were used to estimate the size of hypervariable regions of the gene. Redundant sequences were treated as a single sequence to achieve a better representation of the molecular diversity. Nucleotides found in each position in 95% of the sequences were considered the consensus sequences for different size-groups (consensus95). The sizes of different regions ranged from 96.7 to 283.1 nucleotides and had similar distribution patterns, except for the V3 region, which exhibited a bimodal distribution composed of 2 main peaks of 161 and 186 nt. The alignment of Consensuses95 of fractions 161 and 186 showed a high degree of similarity and conservation, except for the central positions (gap zone), where the sequence was highly variable and several deletions were observed. Structurally, the gap zone forms the central part of helix 17 (H17), and its extension was directly reflected in the size of this helix. H17 is part of a multihelix conjunction known as the 5-way junction (5 WJ), which is indispensable for 30 S ribosome assembly. However, because a drastic variation in the sequence size of V3 region occurs at a central position in loop H17 without affecting the base of the loop, it has no apparent effect on 5 WJ. Finally, considering that these differences were detected in non-redundant sequences, it can be concluded that this is not an uncommon or isolated event and that the V3 region is possibly more likely to mutate than are other regions.

Analysis of SD sequences in completed microbial genomes: non-SD-led genes are as common as SD-led genes

Shine-Dalgarno (SD) sequence has been considered as one of the common features of 5' end untranslated region (5'UTR) of prokaryotic transcripts. However, more leaderless bacteria and archaea mRNAs are being increasingly reported in recent years. To understand the distribution of SD-led genes and non-SD-led genes, we have analyzed 162 completed prokaryotic genomes leading to various new conclusions and validations of previous smaller scale studies. The fact that the number of the SD-led genes among those genomes varies from 11.6% to 90.8% implies that the populations of non-SD-led genes as well as leaderless genes are significant. We found that there is a strong SD conserved region in genomes with high proportion of SD-led genes. Following a t-test we showed that SD sequence content (SDSC) has no correlation with GC content. We observed that the closely related phylogenetic microbes mostly possess a similar SDSC value, and archaeal nonleading genes possess higher SDSC. This study shows that the 5'UTR of prokaryotic genes are highly diverse, particularly when genomes of distantly related organisms are compared, suggesting that more flexible mechanisms are used for translation initiation process in various prokaryotes.