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Blue genes

Date: 2019-06-28 09:26:23.0
Author: Jon Evans

 

Aindrila Mukhopadhyay holding indigoidine powder.

Aindrila Mukhopadhyay holds a vial of purified
indigoidine powder.

Photo: Marilyn Chung/Berkeley Lab.

As any biology textbook will tell you, proteins and peptides are produced by cellular organelles known as ribosomes. But some bacteria and fungi obviously haven’t read this textbook, because they also have recourse to another way. This involves a suite of enzymes able to produce so-called non-ribosomal peptides (NRPs), with a different enzyme required to produce each specific NRP.

These NRPs have a broad range of biological functions, including as toxins and pigments, and so potentially offer a novel route by which microbes can be engineered to produce useful fuels and chemicals. This potential has now been amply demonstrated by a team of US scientists, led by Aindrila Mukhopadhyay at the Lawrence Berkeley National Laboratory in California, through their development of a novel way to color blue jeans.

Mukhopadhyay and her team weren’t intending to transform the blue jean manufacturing process, they were simply exploring whether the genes for the enzymes that produce NRPs, known as NRP synthetases (NRPSs), can be transferred between organisms. To make it easy to tell whether the gene transfer process had worked or not, they chose to transfer a gene for an NRPS that produces a blue pigment called indigoidine. If the transfer worked, then the cells and growth media should turn blue, which would be easy to spot.

Taking a a gene for indigoidine from the soil bacterium Streptomyces lavendulae, they transferred it into a species of red yeast known as Rhodosporidium toruloides. This yeast has recently shown great promise as a microbe for bioproduction, as it can be genetically engineered to produce various useful chemicals at high yields. The transfer worked, producing a strain named BlueBelle that turned itself and its growth media blue.

As they report in a paper in Green Chemistry, Mukhopadhyay and her team next showed that BlueBelle would grow and produce indigoidine on a variety of food sources, including glucose, sucrose, glycerol and xylose, although xylose produced the lowest yields. Most interestingly, they found that BlueBelle would also happily grow on the hydrolysate produced by breaking down plant biomass such as eucalyptus and switchgrass with an ionic liquid, confirming that this approach offers a way to produce useful chemicals from cheap biomass. Indeed, indigoidine may actually turn out to be one of those useful chemicals, as it offers a natural replacement for the synthetic indigo dyes currently used to color jeans.

"Originally extracted from plants, most indigo used today is synthesized," said Mukhopadhyay. "These processes are efficient and inexpensive, but they often require toxic chemicals and generate a lot of dangerous waste. With our work we now have a way to efficiently produce a blue pigment that uses inexpensive, sustainable carbon sources instead of harsh precursors."

This will obviously require scaling up the indigoidine production process, but Mukhopadhyay and her team have already gone some way towards doing this. They developed a batch-fed process that involved breaking down sorghum with an ionic liquid and then growing BlueBelle on the resulting hydrolysate, all in a single 2L bioreactor. Although the process took a bit of time to get going, which the scientists attributed to Bluebelle needing to acclimatize to the bioreactor and the hydrolysate, it eventually produced indigoidine at a rate of almost 3g/L.

When they tried a similar batch process with glucose in a 2L bioreactor, however, the yield increased to 86g/L, which is the highest ever achieved for an NRP. In addition, they showed that the indigoidine could be readily separated from the reaction media and purified.

Mukhopadhyay and her colleagues are already talking with textile companies about using indigoidine in their manufacturing processes and are also investigating what other useful compounds can be produced by transferring NRPS genes into R. toruloides.


The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.


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