Phosphate Starvation Triggers Production and Secretion of an Extracellular Lipoprotein in Caulobacter crescentus

Life in oligotrophic environments necessitates quick adaptive responses to a sudden lack of nutrients. Secretion of specific degradative enzymes into the extracellular medium is a means to mobilize the required nutrient from nearby sources. The aquatic bacterium Caulobacter crescentus must often face changes in its environment such as phosphate limitation. Evidence reported in this paper indicates that under phosphate starvation, C. crescentus produces a membrane surface-anchored lipoprotein named ElpS subsequently released into the extracellular medium. A complete set of 12 genes encoding a type II secretion system (T2SS) is located adjacent to the elpS locus in the C. crescentus genome. Deletion of this T2SS impairs release of ElpS in the environment, which surprisingly remains present at the cell surface, indicating that the T2SS is not involved in the translocation of ElpS to the outer membrane but rather in its release. Accordingly, treatment with protease inhibitors prevents release of ElpS in the extracellular medium suggesting that ElpS secretion relies on a T2SS-secreted protease. Finally, secretion of ElpS is associated with an increase in alkaline phosphatase activity in culture supernatants, suggesting a role of the secreted protein in inorganic phosphate mobilization. In conlusion, we have shown that upon phosphate starvation, C. crescentus produces an outer membrane bound lipoprotein, ElpS, which is further cleaved and released in the extracellular medium in a T2SS-dependent manner. Our data suggest that ElpS is associated with an alkaline phosphatase activity, thereby allowing the bacterium to gather inorganic phosphates from a poor environment

Realization Theory for Deterministic Boundary-Value Descriptor Systems

This paper examines the realization of acausal weighting patterns with two-point boundary-value descriptor systems (TPBVDSs). We restrict our attention to the subclass of TPBVDSs which are extendible, i.e., whose input-output weighting pattern can be extended outwards indefinitely, and stationary, so that their weighting pattern is shift-invariant. Then, given an infinite acausal shift-invariant weighting pattern, the realization problem consists in constructing a minimal TPBVDS over a fixed interval, whose extended weighting pattern matches the given pattern. The realization method which is proposed relies on a new transform, the (s,t) transform, which is used to determine the dimension of a minimal realization, and to construct a minimal realization by factoring two homogeneous rational matrices in the variables s and t. 1