Increased expression of genes involved in uptake and degradation of murein tripeptide under nitrogen starvation in Escherichia coli

Peptidoglycan (also known as murein) is an important envelope component of bacteria, and its turnover usually takes place at considerable levels during normal growth. Amino sugars and murein tripeptide resulting from murein degradation are used for resynthesis of peptidoglycan or as self-generated nutrients or energy sources for cell growth. PgrR (regulator of peptide glycan recycling; formerly YcjZ) was recently identified as a repressor of several genes participating in uptake and degradation of murein tripeptide. In this study, we identified the ycjG gene involved in murein tripeptide degradation as a new direct target of PgrR. The expression of PgrR-regulated genes including ycjY, mppA, mpaA and ycjG was repressed in the presence of a good nitrogen source, but their expression increased under poor nitrogen conditions. Under nitrogen starvation, the pgrR mutant cells exhibited faster growth than wild-type cells, implying that derepression of genes under the control of PgrR may help cells overcome nitrogen limitation. Therefore, these results suggest that nitrogen starvation induces derepression of PgrR-controlled genes involved in uptake and degradation of murein tripeptide, and this may stimulate the utilization of murein tripeptide as a nitrogen source.