Microbiology Editor’s Choice: electron balance in Calvin-cycle mutants

Posted on March 2, 2020   by Microbiology Society

Each month, a manuscript published in our flagship journal Microbiology is chosen by a member of the Editorial Board. This month, the paper is titled “Reductive tricarboxylic acid cycle enzymes and reductive amino acid synthesis pathways contribute to electron balance in a Rhodospirillum rubrum Calvin-cycle mutant” and it was chosen by Professor Gail Preston.

In this paper, the authors investigate how bacteria growing photoheterotrophically (by using light for energy and organic compounds as a source of carbon) in anaerobic conditions are able to maintain electron balance in the absence of respiration. One strategy used by some bacteria is to couple the oxidation of excess reductant to CO₂ fixation via the Calvin cycle. In such cases mutants that lack the Calvin cycle are often unable to grow. Unusually, Calvin-cycle mutants of the purple non-sulfur bacterium Rhodospirillum rubrum retain the ability to grow photoheterotrophically. This study provides intriguing evidence that this could be due to the use of multiple electron management strategies, including the reversal of TCA cycle flux and the use of more reductive biosynthetic pathways, highlighting the flexibility present within bacterial metabolic networks to maintain electron balance.

Reductive tricarboxylic acid cycle enzymes and reductive amino acid synthesis pathways contribute to electron balance in a Rhodospirillum rubrum Calvin-cycle mutant

When growing photosynthetically with organic carbon, purple nonsulfur bacteria canonically avoid a lethal build-up of electrons by reducing CO₂ via the Calvin cycle. Curiously, Calvin-cycle mutants of Rhodospirillum rubrum still grow on compounds with relatively few electrons by an unknown mechanism. By tracking stable isotopes through R. rubrum metabolism, we observed the use of alternative biosynthetic pathways that utilise more electrons more so in the Calvin-cycle mutant compared to the wild type strain. Our results demonstrate that alternative biosynthetic pathways can make an important contribution to electron balance within the constraints of a relatively rigid cellular biomass composition.

We spoke with corresponding author Professor James McKinlay to find out more:

What is your institution and how long have you been there?

I started my lab at Indiana University about eight and-a-half years ago in August 2011.

What is your research area?

Broadly speaking, my research area is bacterial physiology and bacterial interactions. More specifically, my lab seeks to understand how diverse metabolic features of bacteria integrate with cell physiology, a cell’s environment, intra- and inter-species interactions, and biotechnological applications.

What inspired you to research this topic?

We were inspired to research this topic after Gary Roberts’ lab at the University of Wisconsin published results on Rhodospirillum rubrum suggesting that the Calvin cycle might not be required for electron balance in purple nonsulfur bacteria. Their observations went against those I made previously and by several other groups that work with various purple nonsulfur bacteria. We verified the observations by the Roberts lab but found that the results were specific to R. rubrum, raising the question, how does R. rubrum balance electrons without the Calvin cycle?

What is the most rewarding part of your research?

What would you be doing if you weren’t a scientist?

If I weren’t a scientist, I would likely devote more time to illustration, creative writing and music.

Follow Professor McKinlay and co-authors on Twitter:
Alexandra McCully
Maureen Onyeziri
Breah LaSarre
McKinlay Lab

Visit our journals platform to access the full paper. Editor’s Choice articles published in Microbiology are free to read.