Leang Ching, Adams L A, Chin K-J, Nevin K P, Methé B A, Webster J, Sharma M L, Lovley D R
Department of Microbiology, 203 Morrill Science Center IVN, University of Massachusetts at Amherst, Amherst, MA 01003, USA.
J Bacteriol. 2005 Sep;187(17):5918-26. doi: 10.1128/JB.187.17.5918-5926.2005.
Previous studies demonstrated that an outer membrane c-type cytochrome, OmcB, was involved in Fe(III) reduction in Geobacter sulfurreducens. An OmcB-deficient mutant was greatly impaired in its ability to reduce both soluble and insoluble Fe(III). Reintroducing omcB restored the capacity for Fe(III) reduction at a level proportional to the level of OmcB production. Here, we report that the OmcB-deficient mutant gradually adapted to grow on soluble Fe(III) but not insoluble Fe(III). The adapted OmcB-deficient mutant reduced soluble Fe(III) at a rate comparable to that of the wild type, but the cell yield of the mutant was only ca. 60% of that of the wild type under steady-state culturing conditions. Analysis of proteins and transcript levels demonstrated that expression of several membrane-associated cytochromes was higher in the adapted mutant than in the wild type. Further comparison of transcript levels during steady-state growth on Fe(III) citrate with a whole-genome DNA microarray revealed a significant shift in gene expression in an apparent attempt to adapt metabolism to the impaired electron transport to Fe(III). These results demonstrate that, although there are many other membrane-bound c-type cytochromes in G. sulfurreducens, increased expression of these cytochromes cannot completely compensate for the loss of OmcB. The concept that outer membrane cytochromes are promiscuous reductases that are interchangeable in function appears to be incorrect. Furthermore, the results indicate that there may be different mechanisms for electron transfer to soluble Fe(III) and insoluble Fe(III) oxides in G. sulfurreducens, which emphasizes the importance of studying electron transport to the environmentally relevant Fe(III) oxides.
先前的研究表明，外膜c型细胞色素OmcB参与了硫还原地杆菌中Fe(III)的还原过程。一个缺乏OmcB的突变体在还原可溶性和不溶性Fe(III)的能力上都受到了极大的损害。重新引入omcB可恢复Fe(III)还原能力，且恢复水平与OmcB的产生水平成比例。在此，我们报道缺乏OmcB的突变体逐渐适应了在可溶性Fe(III)上生长，但不能在不溶性Fe(III)上生长。适应后的缺乏OmcB的突变体还原可溶性Fe(III)的速率与野生型相当，但在稳态培养条件下，该突变体的细胞产量仅约为野生型的60%。对蛋白质和转录水平的分析表明，在适应后的突变体中，几种与膜相关的细胞色素的表达高于野生型。使用全基因组DNA微阵列进一步比较在柠檬酸铁上稳态生长期间的转录水平，结果显示基因表达发生了显著变化，这显然是为了使代谢适应向Fe(III)的电子传递受损的情况。这些结果表明，尽管硫还原地杆菌中存在许多其他膜结合c型细胞色素，但这些细胞色素表达的增加并不能完全弥补OmcB的缺失。外膜细胞色素是功能上可互换的混杂还原酶这一概念似乎是不正确的。此外，结果表明硫还原地杆菌中向可溶性Fe(III)和不溶性Fe(III)氧化物的电子转移可能存在不同机制，这强调了研究向与环境相关的Fe(III)氧化物的电子传递的重要性。
