Ding Hao, Saer Rafael G, Beatty J Thomas
Department of Microbiology and Immunology, The University of British Columbia, Vancouver, BC, Canada.
Department of Biology, Washington University in St. Louis, St. Louis, MO, United States.
Front Microbiol. 2019 Feb 22;10:301. doi: 10.3389/fmicb.2019.00301. eCollection 2019.
This paper describes a mutant (called SB1707) of the wild type strain SB1003 in which a transposon-disrupted gene resulted in a ∼25-fold increase in the accumulation of coproporphyrin III in the medium of phototrophic (anaerobic) cultures grown in a yeast extract/peptone medium. There was little or no stimulation of pigment accumulation in aerobic cultures. Therefore, this effect of mutation appears to be specific for the anaerobic coproporphyrinogen III oxidase HemN as opposed to the aerobic enzyme HemF. The protein encoded by is homologous to Class I fructose 1,6-bisphosphate aldolases, which catalyze a glycolytic reaction that converts fructose 1, 6-bisphosphate to dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, precursors of pyruvate. There were significant differences in coproporphyrin III accumulation using defined media with individual organic acids and sugars as the sole carbon source: pyruvate, succinate and glutamate stimulated accumulation the most, whereas glucose suppressed coproporphyrin III accumulation to 10% of that of succinate. However, although quantitatively lesser, similar effects of carbon source on the amount of accumulated pigment in the culture medium were seen in a wild type control. Therefore, this mutation appears to exaggerate effects also seen in the wild type strain. It is possible that mutation of causes a metabolic bottleneck or imbalance that was not rectified during growth on the several carbon sources tested. However, we speculate that, analogous to other fructose 1,6-bisphosphate aldolases, the gene product has a "moonlighting" activity that in this case is needed for the maximal expression of the gene. Indeed, it was found that the gene is needed for maximal expression of a promoter- reporter. With the decrease in expression due to the absence of the gene product, coproporphyrinogen III accumulates and is released from the cell, yielding the spontaneous oxidation product coproporphyrin III.
本文描述了野生型菌株SB1003的一个突变体（称为SB1707），其中一个转座子破坏的基因导致在酵母提取物/蛋白胨培养基中进行光合（厌氧）培养的培养基中粪卟啉原III的积累增加了约25倍。在需氧培养中，色素积累几乎没有或没有受到刺激。因此，这种突变效应似乎对厌氧粪卟啉原III氧化酶HemN具有特异性，而不是对需氧酶HemF。编码的蛋白质与I类果糖1,6 - 二磷酸醛缩酶同源，该酶催化将果糖1,6 - 二磷酸转化为二羟基丙酮磷酸和3 - 磷酸甘油醛（丙酮酸的前体）的糖酵解反应。使用以单一有机酸和糖作为唯一碳源的确定培养基时，粪卟啉原III积累存在显著差异：丙酮酸、琥珀酸和谷氨酸对积累的刺激最大，而葡萄糖将粪卟啉原III积累抑制至琥珀酸积累量的10%。然而，尽管数量较少，但在野生型对照中也观察到了碳源对培养基中积累色素量的类似影响。因此，这种突变似乎夸大了在野生型菌株中也能看到的效应。有可能该基因的突变导致了代谢瓶颈或失衡，在测试的几种碳源上生长期间未得到纠正。然而，我们推测，类似于其他果糖1,6 - 二磷酸醛缩酶，该基因产物具有“兼职”活性，在这种情况下，这是该基因最大表达所必需的。事实上，发现该基因是一个启动子 - 报告基因最大表达所必需的。由于该基因产物的缺失导致表达下降，粪卟啉原III积累并从细胞中释放出来，产生自发氧化产物粪卟啉原III。
