Ribosomal mutations affecting the translation of genes that use non-optimal codons.

Genes that are laterally acquired by a new host species often contain codons that are non-optimal to the tRNA repertoire of the new host, which may lead to insufficient translational levels. Inefficient translation can be overcome by different mechanisms, such as incremental amelioration of the coding sequence, compensatory mutations in the regulatory sequences leading to increased transcription or increase in gene copy number. However, there is also a possibility that ribosomal mutations can improve the expression of such genes. To test this hypothesis, we examined the effects of point mutations in the endogenous ribosomal proteins S12 and S5 in Escherichia coli, which are known to be involved in the decoding of the mRNA, on the efficiency of translation of exogenous genes that use non-optimal codons, in vivo. We show that an S12 mutant in E. coli is able to express exogenous genes, with non-optimal codons, to higher levels than the wild-type, and explore the mechanisms underlying this phenomenon in this mutant. Our results suggest that the transient emergence of mutants that allow efficient expression of exogenous genes with non-optimal codons could also increase the chances of fixation of laterally transferred genes.

Menaquinone and iron are essential for complex colony development in Bacillus subtilis.

Cells of undomesticated species of Bacillus subtilis frequently form complex colonies during spreading on agar surfaces. Given that menaquinone is involved in another form of coordinated behavior, namely, sporulation, we looked for a possible role for menaquinone in complex colony development (CCD) in the B. subtilis strain NCIB 3610. Here we show that inhibition of menaquinone biosynthesis in B. subtilis indeed abolished its ability to develop complex colonies. Additionally some mutations of B. subtilis which confer defective CCD could be suppressed by menaquinone derivatives. Several such mutants mapped to the dhb operon encoding the genes responsible for the biosynthesis of the iron siderophore, bacillibactin. Our results demonstrate that both menaquinone and iron are essential for CCD in B. subtilis.

The contribution of common rpsL mutations in Escherichia coli to sensitivity to ribosome targeting antibiotics.

Point mutations in the rpsL gene encoding ribosomal protein S12 can generate resistance to streptomycin, resulting in rapid emergence of resistance to this antibiotic during treatment. In this work, we demonstrate that while spontaneous rpsL mutants in Escherichia coli are resistant to streptomycin, they are more sensitive to the ribosome-targeting antibiotics chloramphenicol, tetracycline and erythromycin. Moreover, combinations of these antibiotics, even in low concentrations were enough to achieve complete growth inhibition of both wild type and rpsL mutant strains. Thus, combining ribosome-targeting drugs can be used as a new treatment strategy that may be effective against streptomycin-resistant ribosome mutants.

Effect of ribosome-targeting antibiotics on streptomycin-resistant Mycobacterium mutants in the rpsL gene.

Streptomycin (Sm) was the first antibiotic used against Mycobacterium tuberculosis, the aetiological agent of tuberculosis (TB). However, point mutations in the rpsL gene can generate resistance to Sm, which is why spontaneous resistance to this antibiotic emerges so rapidly during treatment. Here we examine the interaction between Sm resistance and sensitivity to other ribosome-targeting antibiotics. Levels of resistance of rpsL mutants to the ribosome-affecting antibiotics chloramphenicol, tetracycline, gentamicin and erythromycin were tested, both singly and in combination. For this purpose, Mycobacterium smegmatis was used, which is commonly used in laboratory experiments as a model for TB. Generally, Sm-resistant mutants were as sensitive to the ribosome-affecting antibiotics as the wild-type strain. Combinations of different ribosome-affecting antibiotics were occasionally more potent than either of the single drugs, with better inhibition of both wild-type and mutant strains. Combining different ribosome-affecting drugs could represent an additional strategy in treating mycobacterial infections, including those resistant to newer drugs such as isoniazid or ethambutol.