||To understand how cell physiological state affects mRNA translation, we used Shewanella oneidensis MR-1 grown under steady state conditions at either 20% or 8.5% O-2. Using a combination of quantitative proteomics and RNA-Seq, we generated high-confidence data on gt; 1000 mRNA and protein pairs. By using a steady state model, we found that differences in protein-mRNA ratios were primarily due to differences in the translational efficiency of specific genes. When oxygen levels were lowered, 28% of the proteins showed at least a 2-fold change in expression. Transcription levels were sp. significantly altered for 26% of the protein changes; translational efficiency was significantly altered for 46% and a combination of both was responsible for the remaining 28%. Changes in translational efficiency were significantly correlated with the codon usage pattern of the genes and measurable tRNA pools changed in response to altered O-2 levels. Our results suggest that changes in the translational efficiency of proteins, in part due to altered tRNA pools, is a major determinant of regulated alterations in protein expression levels in bacteria.