Essential is not irreplaceable: fitness dynamics of experimental E. coli RNase P RNA heterologous replacement.

While critical cellular components-such as the RNA moiety of bacterial ribonuclease P-can sometimes be replaced with a highly divergent homolog, the cellular response to such perturbations is often unexpectedly complex. RNase P is a ubiquitous and essential ribonucleoprotein involved in the processing of multiple RNA substrates, including tRNAs, small non-coding RNAs and intergenic operons. In Bacteria, RNase P RNAs have been subdivided-based on their secondary and tertiary structures-into two major groups (A and B), each with a distinct phylogenetic distribution. Despite the vast phylogenetic and structural gap that separates the two RNase P RNA classes, previous work suggested their interchangeability. Here, we explore in detail the functional and fitness consequences of replacing the endogenous Type-A Escherichia coli RNase P RNA with a Type-B homolog derived from Bacillus subtilis, and show that E. coli cells forced to survive with a chimeric RNase P as their sole source of RNase P activity exhibit extremely variable responses. The chimeric RNase P alters growth rates-used here as an indirect measure of fitness-in unpredictable ways, ranging from 3- to 20-fold reductions in maximal growth rate. The transcriptional behavior of cells harboring the chimeric RNAse P is also perturbed, affecting the levels of at least 79 different transcripts. Such transcriptional plasticity represents an important mechanism of transient adaptation which, when coupled with the emergence and eventual fixation of compensatory mutations, enables the cells to overcome the disruption of this tightly coevolving ribonucleoprotein.