A horizontally acquired tRNA facilitates Coxiella burnetii adaptation to an extreme environment.

2014-08-24T18:55:00Z (GMT) by Jess Millar Rahul Raghavan
<p><em>Coxiella burnetii</em> is an obligate intracellular bacterium that lives inside parasitophorous vacuoles (PV) derived from lysosomes. The PV is acidic (~4.5 pH) and <em>Coxiella</em> is the only bacterium known to thrive in this extreme environment. However, the metabolic adaptations that allow <em>Coxiella</em> to replicate in this niche are unknown. <em>Coxiella</em> has a highly reduced genome due to lack of selection pressure to maintain superfluous genes; however, it has retained an extra copy of tRNA<em>Glu</em>. Horizaontal gene transfer is the likely origin of the additional isoacceptor (tRNA<em>Glu</em>2) present in <em>Coxiella</em> as it is not present in other Gammaproteobacteria. In this study, <em>C. burnetii</em> was grown in THP-1 cells and High Throughput Sequencing was used to examine tRNA isoacceptors and measure gene expression values for all genes. All heme biosynthesis genes and tRNA<em>Glu</em>2 were highly expressed in <em>Coxiella</em> during intracellular growth. <em>Coxiella</em> lacks heme transporters but has ferrous ion transporters. This is the major iron form found in the highly acidic PV, and the form used in heme biosynthesis. Additionally, tRNA<em>Glu</em>2 can only decode the GAG codon but the prevalence of GAA codons in proteins was three times higher than that of GAG codons. tRNA<em>Glu</em> not only has a role in protein biosynthesis, but it’s also the starting point for heme biosynthesis. Many bacteria obtain heme from their environment using heme transporters, however these are missing in <em>Coxiella</em>. The high expression of both heme biosynthesis genes and tRNA<em>Glu</em>2 highlights its importance, likely due to heme’s vital role in respiration and other processes. With the prevalence of GAA codons three times that of GAG, which only tRNA<em>Glu</em>1 can decode, we hypothesize that the primary role of tRNA<em>Glu</em>2 in <em>Coxiella</em> is heme biosynthesis and not translation. Our next steps will be to construct gene deletion strains of <em>Coxiella</em> and examine the importance of tRNA<em>Glu</em>2 and heme biosynthesis for <em>Coxiella</em>’s intracellular growth and virulence. The first two enzymes (HemA and HemL) in <em>Coxiella</em>’s heme biosynthesis process are not found in humans; hence they are attractive targets for developing novel antibiotics to treat <em>Coxiella</em>. This is significant as current antibiotics are not very effective against chronic endocarditis caused by <em>Coxiella</em> and resistance to these antibiotics is prevalent among clinical strains.</p>