TheCaulobacterNtrB-NtrC two-component system bridges nitrogen assimilation and cell development

Abstract

A suite of molecular sensory systems enablesCaulobacterto control growth, development, and reproduction in response to levels of essential elements. The enhancer binding protein NtrC and its cognate sensor histidine kinase NtrB are well-established regulators of nitrogen assimilation in bacteria, but their precise functions inCaulobactermetabolism and cell development remain largely undefined. Deletion ofC. crescentus ntrCslowed cell growth in complex medium, whilentrBandntrCwere essential for growth when ammonium was the sole nitrogen source due to their requirement for glutamine synthase (glnA) expression. Random transposition of a conserved IS3-family mobile genetic element frequently rescued the growth defect ofntrCmutant strains by restoring transcription of theglnBAoperon, revealing a possible role for IS3 transposition in shaping the evolution ofCaulobacterpopulations during nitrogen limitation. The chromosome ofC. crescentusharbors dozens of NtrC binding sites, with a large fraction located near genes involved in polysaccharide biosynthesis. The majority of NtrC binding sites align with those of the nucleoid associated protein, GapR, an essential protein involved in chromosome organization, or MucR1, a cell cycle regulator. Thus, NtrC is predicted to directly impact the regulation of cell cycle and cell development. Indeed, loss of NtrC function led to elongated polar stalks and elevated synthesis of cell envelope polysaccharides. These phenotypes were rescued by supplementing media with glutamine or by ectopic expression of theglnBAoperon. This study establishes regulatory connections between NtrC, nitrogen metabolism, polar morphogenesis, and envelope polysaccharide synthesis inCaulobacter.