Miller S R, Castenholz R W
Department of Biology, University of Oregon, Eugene, Oregon 97403, USA.
Appl Environ Microbiol. 2001 Jul;67(7):3002-9. doi: 10.1128/AEM.67.7.3002-3009.2001.
Synechococcus sp. strain SH-94-5 is a nitrate assimilation-deficient cyanobacterium which was isolated from an ammonium-replete hot spring in central Oregon. While this clone could grow on ammonium and some forms of organic nitrogen as sole nitrogen sources, it could not grow on either nitrate or nitrite, even under conditions favoring passive diffusion. It was determined that this clone does not express functional nitrate reductase or nitrite reductase and that the lack of activity of either enzyme is not due to inactivation of the cyanobacterial nitrogen control protein NtcA. A few other naturally occurring cyanobacterial strains are also nitrate assimilation deficient, and phylogenetic analyses indicated that the ability to utilize nitrate has been independently lost at least four times during the evolutionary history of the cyanobacteria. This phenotype is associated with the presence of environmental ammonium, a negative regulator of nitrate assimilation gene expression, which may indicate that natural selection to maintain functional copies of nitrate assimilation genes has been relaxed in these habitats. These results suggest how the evolutionary fates of conditionally expressed genes might differ between environments and thereby effect ecological divergence and biogeographical structure in the microbial world.
聚球藻属菌株SH-94-5是一种缺乏硝酸盐同化能力的蓝细菌，它是从俄勒冈州中部一个富含铵的温泉中分离出来的。虽然该克隆体能够以铵和某些形式的有机氮作为唯一氮源生长，但即使在有利于被动扩散的条件下，它也不能在硝酸盐或亚硝酸盐上生长。经测定，该克隆体不表达功能性硝酸还原酶或亚硝酸还原酶，且这两种酶缺乏活性并非由于蓝细菌氮控制蛋白NtcA失活所致。其他一些天然存在的蓝细菌菌株也缺乏硝酸盐同化能力，系统发育分析表明，在蓝细菌的进化历史中，利用硝酸盐的能力至少独立丧失了四次。这种表型与环境铵的存在有关，铵是硝酸盐同化基因表达的负调节因子，这可能表明在这些生境中，维持硝酸盐同化基因功能拷贝的自然选择已经放松。这些结果表明，条件表达基因的进化命运在不同环境中可能存在差异，从而影响微生物世界中的生态分化和生物地理结构。
