Neidle E L, Kaplan S
Department of Microbiology and Molecular Genetics, University of Texas Health Science Center, Houston 77225.
J Bacteriol. 1993 Apr;175(8):2304-13. doi: 10.1128/jb.175.8.2304-2313.1993.
In the photosynthetic bacterium Rhodobacter sphaeroides, two genes, hemA and hemT, each encode a distinct 5-aminolevulinic acid (ALA) synthase isozyme (E. L. Neidle and S. Kaplan, J. Bacteriol. 175:2292-2303, 1993). This enzyme catalyzes the first and rate-limiting step in a branched pathway for tetrapyrrole formation, leading to the biosynthesis of hemes, bacteriochlorophylls, and corrinoids. In an attempt to determine the functions of hemA and hemT, mutant strains were constructed with specific chromosomal disruptions. These chromosomal disruption allowed hemA and hemT to be precisely localized on the larger and smaller of two R. sphaeroides chromosomes, respectively. Mutants carrying a single hemA or hemT disruption grew well without the addition of ALA, whereas a mutant, HemAT1, in which hemA and hemT had both been inactivated required exogenous ALA for growth. The growth rates, ALA synthase enzyme levels, and the amounts of bacteriochlorophyll-containing intracytoplasmic membrane spectral complexes of all strains were compared. Under photosynthetic growth conditions, the levels of bacteriochlorophyll, carotenoids, and B800-850 and B875 light-harvesting complexes were significantly lower in the Hem mutants than in the wild type. In the mutant strains, available bacteriochlorophyll appeared to be preferentially targeted to the B875 light-harvesting complex relative to the B800-850 complex. In strain HemAT1, the amount of B800-850 complex varied with the concentration of ALA added to the growth medium, and under conditions of ALA limitation, no B800-850 complexes could be detected. In the Hem mutants, there were aberrant transcript levels corresponding to the puc and puf operons encoding structural polypeptides of the B800-850 and B875 complexes. These results suggest that hemA and hemT expression is coupled to the genetic control of the R. sphaeroides photosynthetic apparatus.
在光合细菌球形红杆菌中，hemA和hemT这两个基因各自编码一种独特的5-氨基乙酰丙酸（ALA）合酶同工酶（E.L.尼德尔和S.卡普兰，《细菌学杂志》175:2292 - 2303，1993年）。这种酶催化四吡咯形成分支途径中的第一步且是限速步骤，该途径导致血红素、细菌叶绿素和类咕啉的生物合成。为了确定hemA和hemT的功能，构建了具有特定染色体缺失的突变菌株。这些染色体缺失使得hemA和hemT分别精确地定位在球形红杆菌两条染色体中较大和较小的染色体上。携带单个hemA或hemT缺失的突变体在不添加ALA的情况下生长良好，而hemA和hemT均已失活的突变体HemAT1则需要外源ALA才能生长。比较了所有菌株的生长速率、ALA合酶酶水平以及含细菌叶绿素的胞内膜光谱复合物的量。在光合生长条件下，Hem突变体中细菌叶绿素、类胡萝卜素以及B800 - 850和B875光捕获复合物的水平显著低于野生型。在突变菌株中，相对于B800 - 850复合物，可用的细菌叶绿素似乎优先靶向B875光捕获复合物。在菌株HemAT1中，B800 - 850复合物的量随添加到生长培养基中的ALA浓度而变化，并且在ALA受限的条件下，检测不到B800 - 850复合物。在Hem突变体中，存在与编码B800 - 850和B875复合物结构多肽的puc和puf操纵子相对应的异常转录水平。这些结果表明hemA和hemT的表达与球形红杆菌光合装置的遗传控制相关联。
