Proteomic identification of tmRNA substrates.

The tmRNA-SmpB system releases ribosomes stalled on truncated mRNAs and tags the nascent polypeptides to target them for proteolysis. In many species, mutations that disrupt tmRNA activity cause defects in growth or development. In Caulobacter crescentus cells lacking tmRNA activity there is a delay in the initiation of DNA replication, which disrupts the cell cycle. To understand the molecular basis for this phenotype, 73 C. crescentus proteins were identified that are tagged by tmRNA under normal growth conditions. Among these substrates, proteins involved in DNA replication, recombination, and repair were overrepresented, suggesting that misregulation of these factors in the absence of tmRNA activity might be responsible for the delay in initiation of DNA replication. Analysis of the tagging sites within these substrates revealed a conserved nucleotide motif 5' of the tagging site, which is required for wild-type tmRNA tagging.