Inteins are self-splicing polypeptides with an ability to excise themselves from flanking host protein regions with remarkable precision; in the process, they ligate flanked host protein fragments. Inteins are distributed sporadically across all three domains of life (bacteria, archaea, and unicellular eukaryotes). However, their apparent localized distribution in DNA replication, repair, and recombination proteins (the 3Rs), particularly in bacteria and archaea, is enigmatic. Our understanding of the localized distribution of inteins in the 3Rs, and their possible regulatory role in such distribution, is still only partial. Nevertheless, understanding the chemistry of post-translational self-splicing of inteins has opened up opportunities for protein chemists to modify, manipulate, and bioengineer proteins. Protein-splicing technology is adapted to a wide range of applications, starting with untagged protein purification, site-specific protein labeling, protein biotinylation, isotope incorporation, peptide cyclization, as an antimicrobial target, and so on. This review is focused on the chemistry of splicing; the localized distribution of inteins, particularly in the 3Rs and their possible role in regulating host protein function; and finally, the use of protein-splicing technology in various protein engineering applications.

3' → 5' exoribonucleases play a critical role in many aspects of RNA metabolism. RNase R, PNPase, and RNase II are the major contributors to RNA processing, maturation, and quality control in bacteria. Bacteria don't seem to have dedicated RNA degradation machineries to process different classes of RNAs. Under different environmental and physiological conditions, their roles can be redundant and sometimes overlapping. Here, I discuss why PNPase and RNase R may have switched their physiological roles in some bacterial species to adapt to environmental conditions, despite being biochemically distinct exoribonucleases.

Low temperatures constrain cellular life due to reductions in nutrient uptake, enzyme kinetics, membrane permeability, and function of other biomacromolecules. This has implications for the biophysical limits of life on Earth, and the plausibility of life in extraterrestrial locations. Although most pseudomonads are mesophilic in nature, isolates such as the Antarctic Pseudomonas syringae Lz4W exhibit considerable psychrotolerance, with an ability to grow even between 4 and 0°C. In this review, we explore the molecular traits and characteristic phenotypes of P. syringae Lz4W that enable life at low temperatures. We describe adaptations that enhance membrane fluidity; examine genes involved in cellular function and survival in the cold; assess capability for energy generation at low temperature; and detail the mechanics of DNA repair and RNA processing at low temperature, and speculate that P. syringae Lz4W can also synthesize glycerol to maintain flexibility of macromolecular systems. In the range 4 to 0ºC, there are considerable changes in the properties and behaviour of water. Specifically, density can have adverse impacts on plasma-membrane functions, cytoplasmic viscosity, protein behaviour, and other essential properties of cellular system. We identified a combination of adaptations that may be peculiar to cold-tolerant P. syringae, including increase of unsaturated fatty acids in the plasma membrane; a RNA polymerase able to function at 0°C; RecBCD- and RuvAB-dependent reestablishment of replication fork; and efficiencies of degradosome machinery and RNA processing by RNaseR at low temperature. Several unresolved questions are discussed in the context of astrobiology, and further work needed on the psychrotolerance of P. syringae.

Pseudomonas syringae Lz4W RecBCD enzyme, RecBCDPs, is a trimeric protein complex comprised of RecC, RecB, and RecD subunits. RecBCD enzyme is essential for P. syringae growth at low temperature, and it protects cells from low temperature induced replication arrest. In this study, we show that the RecBCDPs enzyme displays distinct biochemical behaviors. Unlike E. coli RecBCD enzyme, the RecD subunit is indispensable for RecBCDPs function. The RecD motor activity is essential for the Chi-like fragments production in P. syringae, highlighting a distinct role for P. syringae RecD subunit in DNA repair and recombination process. Here, we demonstrate that the RecBCDPs enzyme recognizes a unique octameric DNA sequence, 5'-GCTGGCGC-3' (ChiPs) that attenuates nuclease activity of the enzyme when it enters dsDNA from the 3'-end. We propose that the reduced translocation activities manifested by motor-defective mutants cause cold sensitivity in P. syrinage; emphasizing the importance of DNA processing and recombination functions in rescuing low temperature induced replication fork arrest.

The loading of RecA onto ssDNA by RecBCD is an essential step of RecBCD-mediated homologous recombination. RecBCD facilitates RecA-loading onto ssDNA in a χ-dependent manner via its RecB nuclease domain (RecBn). Before recognition of χ, RecBn is sequestered through interactions with RecBCD. It was proposed that upon χ-recognition, RecBn undocks, allowing RecBn to swing out via a contiguous 70 amino acid linker to reveal the RecA-loading surface, and then recruit and load RecA onto ssDNA. We tested this hypothesis by examining the interactions between RecBn (RecB928-1180) and truncated RecBCD (RecB1-927CD) lacking the nuclease domain. The reconstituted complex of RecB1-927CD and RecBn is functional in vitro and in vivo. Our results indicate that despite being covalently severed from RecB1-927CD, RecBn can still load RecA onto ssDNA, establishing that RecBn does not function while only remaining tethered to the RecBCD complex via the linker. Instead, RecBCD undergoes a χ-induced intramolecular rearrangement to reveal the RecA-loading surface.

RNase R (encoded by the rnr gene) is a highly processive 3' → 5' exoribonuclease essential for the growth of the psychrotrophic bacterium P. syringae Lz4W at low temperature. The cell death of a rnr deletion mutant at low temperature has been previously attributed to the processing defects in 16S rRNA, defective ribosomal assembly and inefficient protein synthesis. We recently showed that RNase R is required to protect P. syringae Lz4W from DNA damage and oxidative stress, independent of its exoribonuclease activity. Here, we show that the processing defect in 16S rRNA does not cause cell death of the rnr mutant of P. syringae at low temperature. Our results demonstrate that the rnr mutant of P. syringae Lz4W, complemented with a RNase R deficient in exoribonuclease function (RNase R^D284A) is defective in 16S rRNA processing but can grow at 4 ^oC. This suggested that the processing defect in ribosomal RNAs is not a cause of the cold sensitivity of the rnr mutant. We further show that the rnr mutant accumulates copies of the indigenous plasmid of P. syringae Lz4W, pLz4W, that bears a type II toxin-antitoxin system (psA-psT). This phenotype was rescued by over-expressing antitoxin psA in the rnr mutant, suggesting that activation of the type II toxin-antitoxin system leads to cold sensitivity of the rnr mutant of P. syringae Lz4W. Here, we report a previously unknown functional relationship between the cold sensitivity of the rnr mutant and a type II toxin-antitoxin system in P. syringae Lz4W.