Epsilon Open Archive

Abstract

Initiation of chromosome replication is a tightly controlled process. We have developed a stochastic model with all known major features of the regulation of the initiation in Escherichia coli, which automatically generates the correct initiation frequency and chromosome number, synchronous initiation of multiple origins and increasing cell size with increasing cell growth rate. We further suggest a principle for how an initiator may at the same time adequately regulate its own gene expression. In Eubacteria, macrolide antibiotics bind at the entrance of the nascent peptide exit tunnel of the large ribosomal subunit and induce premature termination of translation by drop-off of peptidyl-tRNA. We have combined biochemical experiments and in vivo growth experiments with modelling to study peptide-mediated resistance against two different macrolides, erythromycin and josamycin. The mechanism behind resistance is different for the two drugs. In the case of erythromycin, we have verified that synthesis of a cis-acting peptide accelerates the rate of dissociation of erythromycin. In the case of josamycin, the drop-off rate of peptidyl-tRNA is considerably slowed down for a peptide sequence mediating resistance in relation to a control peptide sequence. We also show that peptide-mediated resistance requires the AcrAB-TolC efflux system in the cell membrane. The most widely spread resistance mechanism against macrolides are a modification of the 23S rRNA by an Erm methylase which considerably lowers the affinity of macrolides to the ribosome. Induction of erythromycin resistance by ErmC requires ribosomes both with and without erythromycin. Modelling suggests that there exists a maximal induction rate of the synthesis of ErmC and resistant ribosomes at a certain concentration of the antibiotic. We also derive how bi-stable growth rates may arise for bacterial cells growing exponentially at a fixed external antibiotic concentration as a consequence of low membrane permeability of the drug and high growth rate sensitivity to the intracellular concentration of the drug concentration.