Yersinia enterocolitica : Genes involved in cold-adaptation

Abstract

It is known from the literature that:

-The application of chilling as a means of food preservation has frequently resulted in food borne infections with psychrotrophic micro-organisms, such as Yersinia enterocolitica, Listeria monocytogenes and Aeromonas hydrophila; - The injurious effect on human health of an infection with Y. enterocolitica should not be underestimated because of the risk of serious post-infective complications; - Almost all micro-organisms respond to an abrupt temperature down shift (within their growth range) by a temporary cessation of growth, and the concomitant transient synthesis of a set of specific proteins, called 'cold-shock-proteins'; - Many psychrotrophic micro-organisms synthesize at low temperatures a set of proteins, called 'cold-acclimation-proteins', whose expression is either enhanced or specifically induced at these temperatures; - To explain the induction and function of the cold-shock response, a model called the Cold-Shock Ribosomal Adaptation (CSRA-) model has been proposed; - This model comprises four steps, i.e. (1) cancelling of the regular translation of 'household' gene mRNA's, in favour of the translation of cold-shock gene mRNA's, (2) specific modification of the ribosome by the cold-shock proteins, (3) the re-initiation of regular mRNA translation, and (4) the concomitant down regulation of the expression of cold-shock genes; - The CSRA-model does not explain why the production of cold-shock and/or cold acclimation proteins does not cease in psychrotrophs; - The inability of mesophiles to sustain growth at temperatures below 7 degrees C has been explained as a result of insufficient cold-shock response, compared to psychrotrophs; -Polynucleotide phosphorylase (PNPase) , one of the two essential 3'-5'exoribonucleases in E. coli, is a cold-shock protein, but the gene is not cold-inducible by its own promoter. The other 3'-5'exoribonuclease, RNaseII, is not cold induced; - PNPase is able to interact with the ribosome, and PNPase is better at the degrading of mRNA's which contain secondary structures, compared to its counterpart RNaseII; - PNPase uses a relatively energy-saying mode of mRNA degradation, compared to RNaseII, and has in that way a central position in the efficient turnover of the essential DNA-precursor cytosine diphosphate (CDP) ; - Energy levels are growth limiting for E, coli at low temperatures. It has been shown in the present study that:

The emergence of Y. enterocolitica as a human pathogen in our era can at least partly be explained as a result of the enormously increased application of refrigeration in the preservation of foods; - The ability of Y. enterocolitica to grow at refrigerator temperatures is dependent on expression of the bacterial pnp gene, encoding the exoribonuclease polynucleotide phosphorylase (PNPase); The cellular concentration of PNPase in Y. enterocolitica is increased during growth at refrigeration temperatures; -. The cold-induced rise of the PNPase concentration in Yersinia enterocolitica is accompanied by an increase in pnp mRNA, and this is mediated by a cold-inducible promoter of the pnp gene; - A similar cold-induced increase in PNPase concentration occurs in all other, invariably psychrotrophic, species of the genus Yersinia; - In all other Yersinia species, the organization of the regulatory regions of the pnp gene is identical to that in Y. enterocolitica. It is concluded that:

PNPase is an essential factor for adaptation to reduced temperatures, presumably not only in Y. enterocolitica, but also in other, mesophilic bacteria; Due to its ability to interact with ribosomes, PNPase fits well in the CSRA-model. Its mode of action may be in the enhanced decay of those mRNA's which have lost their polysomes, resulting in a more efficient synthesis as well as in increased translation of the mRNA's of the cold-shock genes; - . According to this function in the CSRA-model, PNPase might be a restrictive factor for efficient cold-adaptation. Consequently, an insufficient enhancement at 7 degrees C or below, or a subsequent decline of PNPase expression, might be the cause of the definitive cessation of growth of mesophiles at this temperature; - In view of the cellular difficulties in liberating sufficient energy under cold-stress conditions, it is not unlikely that the lower temperature limit for growth is set by the cellular capacity to maintain the catabolism under these conditions; - On behalf of its energy-saving mode of mRNA decay, PNPase could be the key factor that is needed under energy-limiting conditions to provide the cell with the obligatory DNA-precursor cytosine diphosphate (CDP) ; - Further studies are needed to unravel how pnp-expression is regulated and what role is played by PNPase in the process of cold-adaptation, in order to elucidate the mechanism(s) which set the lower temperature limit for growth of micro-organisms.