Various checkpoints prevent the synthesis of Staphylococcus aureus peptidoglycan hydrolase LytM in the stationary growth phase

In Staphylococcus aureus, peptidoglycan metabolism plays a role in the host inflammatory response and pathogenesis. Transcription of the peptidoglycan hydrolases is activated by the essential 2-component system WalKR at low cell density. During stationary growth phase, WalKR is not active and transcription of the peptidoglycan hydrolase genes is repressed. In this work, we studied regulation of expression of the glycylglycine endopeptidase LytM. We show that, in addition to the transcriptional regulation mediated by WalKR, the synthesis of LytM is negatively controlled by a unique mechanism at the stationary growth phase. We have identified 2 different mRNAs encoding lytM, which vary in the length of their 5' untranslated (5'UTR) regions. LytM is predominantly produced from the WalKR-regulated mRNA transcript carrying a short 5'UTR. The lytM mRNA is also transcribed as part of a polycistronic operon with the upstream SA0264 gene and is constitutively expressed. Although SA0264 protein can be synthesized from the longer operon transcript, lytM cannot be translated because its ribosome-binding site is sequestered into a translationally inactive secondary structure. In addition, the effector of the agr system, RNAIII, can inhibit translation of lytM present on the operon without altering the transcript level but does not have an effect on the translation of the upstream gene. We propose that this dual regulation of lytM expression, at the transcriptional and post-transcriptional levels, contributes to prevent cell wall damage during the stationary phase of growth.

Using Vaccinia's innate ability to introduce DNA into mammalian cells for production of recombinant proteins

Production of recombinant protein in mammalian cells is time-consuming, labor-intensive and costly. While seeking to overcome these limitations, we discovered that Vaccinia virus has the innate ability to transfer exogenous plasmid DNA into mammalian cells during the infection process. Parameters influencing the efficiency of this event were characterized and a quick, simple and inexpensive way to produce eukaryotic proteins was established.

Stationary-phase expression and aminoacylation of a transfer-RNA-like small RNA

Genome-scale analyses have shown numerous functional duplications in the canonical translational machinery. One of the most striking examples is the occurrence of unrelated class I and class II lysyl-transfer RNA synthetases (LysRS), which together may aminoacylate non-canonical tRNAs. We show that, in Bacillus cereus, the two LysRSs together aminoacylate a small RNA of unknown function named tRNA(Other), and that the aminoacylated product stably binds translation elongation factor Tu. In vitro reconstitution of a defined lysylation system showed that Lys-tRNA(Other) is synthesized in the presence of both LysRSs, but not by either alone. In vivo analyses showed that the class 2 LysRS was present both during and after exponential growth, whereas the class I enzyme and tRNA(Other) were predominantly produced during the stationary phase. Aminoacylation of tRNA(Other) was also found to be confined to the stationary phase, which suggests a role for this non-canonical tRNA in growth-phase-specific protein synthesis.

Nonorthologous replacement of lysyl-tRNA synthetase prevents addition of lysine analogues to the genetic code

Insertion of lysine during protein synthesis depends on the enzyme lysyl-tRNA synthetase (LysRS), which exists in two unrelated forms, LysRS1 and LysRS2. LysRS1 has been found in most archaea and some bacteria, and LysRS2 has been found in eukarya, most bacteria, and a few archaea, but the two proteins are almost never found together in a single organism. Comparison of structures of LysRS1 and LysRS2 complexed with lysine suggested significant differences in their potential to bind lysine analogues with backbone replacements. One such naturally occurring compound, the metabolic intermediate S-(2-aminoethyl)-L-cysteine, is a bactericidal agent incorporated during protein synthesis via LysRS2. In vitro tests showed that S-(2-aminoethyl)-L-cysteine is a poor substrate for LysRS1, and that it inhibits LysRS1 200-fold less effectively than it inhibits LysRS2. In vivo inhibition by S-(2-aminoethyl)-L-cysteine was investigated by replacing the endogenous LysRS2 of Bacillus subtilis with LysRS1 from the Lyme disease pathogen Borrelia burgdorferi. B. subtilis strains producing LysRS1 alone were relatively insensitive to growth inhibition by S-(2-aminoethyl)-L-cysteine, whereas a WT strain or merodiploid strains producing both LysRS1 and LysRS2 showed significant growth inhibition under the same conditions. These growth effects arising from differences in amino acid recognition could contribute to the distribution of LysRS1 and LysRS2 in different organisms. More broadly, these data demonstrate how diversity of the aminoacyl-tRNA synthetases prevents infiltration of the genetic code by noncanonical amino acids, thereby providing a natural reservoir of potential antibiotic resistance.