Relating mRNA and protein biomarker levels in a Dehalococcoides and Methanospirillum-containing community

To better understand the quantitative relationships between messenger RNA (mRNA) and protein biomarkers relevant to bioremediation, we quantified and compared respiration-associated gene products in an anaerobic syntrophic community. Respiration biomarkers for Dehalococcoides, an organohalide reducer, and Methanospirillum, a hydrogenotrophic methanogen, were quantified via qRT-PCR for mRNA and multiple reaction monitoring (MRM) of proteotypic peptides for protein. mRNA transcripts of the Dehalococcoides reductive dehalogenases PceA, TceA, and DMC1545, and hydrogenase HupL, as well as the Methanospirillum oxidoreductases MvrD and FrcA were shown to be similarly regulated with respect to their temporal responses to substrate addition. However, MvrD was two orders of magnitude lower in mRNA abundance. Per cell, Dehalococcoides protein biomarkers quantified were more abundant than Methanospirillum proteins. Comparing mRNA with protein abundance, poor correlations were observed between mRNA transcript levels and the net protein produced. For example, Dehalococcoides HupL and TceA transcripts were similarly abundant though TceA was far more abundant at the protein level (167 ± 121 vs. 1095 ± 337 proteins per cell, respectively). In Methanospirillum, MvrD maintained comparable per-cell protein abundance to FrcA (42 ± 14 vs. 60 ± 1 proteins per cell, respectively) despite the significantly lower transcript levels. Though no variability in protein decay rates was observed, the mRNA translation rate quantified for TceA was greater than the other Dehalococcoides targets monitored. These data suggest that there is considerable variation in the relationship between mRNA abundance and protein production both across transcripts within an organism and across organisms. This highlights the importance of empirically based studies for interpreting biomarker levels in environmentally relevant organisms.