Houng H S, Kopecko D J, Baron L S
Department of Bacterial Immunology, Walter Reed Army Institute of Research, Washington, D.C. 20307.
J Bacteriol. 1990 Aug;172(8):4392-8. doi: 10.1128/jb.172.8.4392-4398.1990.
The chromosomally encoded galactose utilization (gal) operons of Salmonella typhimurium and S. typhi were each cloned on similar 5.5-kilobase HindIII fragments into pBR322 and were identified by complementation of Gal- Escherichia coli strains. Restriction endonuclease analyses indicated that these Salmonellae operons share considerable homology, but some heterogeneities in restriction sites were observed. Subcloning and exonuclease mapping experiments showed that both operons have the same genetic organization as that established for the E. coli gal operon (i.e., 5' end, promoter, epimerase, transferase, kinase, and 3' end). Two gal operator regions (oE and oI) of S. typhimurium, identified by repressor titration in an E. coli superrepressor [galR(Sup)] mutant, were sequenced and found to flank the promoter region. This promoter region is identical to the -10 and -35 regions of the E. coli gal operon. Minicell studies demonstrated that the three gal structural genes of S. typhimurium encode separate polypeptides of 39 kilodaltons (kDa) (epimerase, 337 amino acids [aa's]), 41 kDa (transferase, 348 aa's), and 43 kDa (kinase, 380 aa's). Despite functional and organizational similarities, DNA sequence analysis revealed that the S. typhimurium gal genes show less than 70% homology to the E. coli gal operon. Because of codon degeneracy, the deduced amino acid sequences of these polypeptides are highly conserved (greater than 90% homology) as compared with those of the E. coli gal enzymes. These studies have defined basic genetic parameters of the gal genes of two medically important Salmonella species, and our findings support the hypothesized divergent evolution of E. coli and Salmonella spp. from a common ancestral parent bacterium.
鼠伤寒沙门氏菌和伤寒沙门氏菌的染色体编码半乳糖利用（gal）操纵子分别被克隆到相似的5.5千碱基HindIII片段上，并插入pBR322中，通过对Gal⁻大肠杆菌菌株的互补作用进行鉴定。限制性内切酶分析表明，这些沙门氏菌操纵子具有相当大的同源性，但在限制性酶切位点上观察到一些异质性。亚克隆和外切核酸酶图谱实验表明，这两个操纵子与已确定的大肠杆菌gal操纵子具有相同的遗传组织（即5'端、启动子、差向异构酶、转移酶、激酶和3'端）。通过在大肠杆菌超阻遏物[galR（Sup）]突变体中进行阻遏物滴定鉴定出的鼠伤寒沙门氏菌的两个gal操纵基因区域（oE和oI）被测序，发现它们位于启动子区域两侧。该启动子区域与大肠杆菌gal操纵子的-10和-35区域相同。小细胞研究表明，鼠伤寒沙门氏菌的三个gal结构基因编码分别为39千道尔顿（kDa）（差向异构酶，337个氨基酸[aa]）、41 kDa（转移酶，348个aa）和43 kDa（激酶，380个aa）的单独多肽。尽管在功能和组织上有相似性，但DNA序列分析表明，鼠伤寒沙门氏菌的gal基因与大肠杆菌gal操纵子的同源性低于70%。由于密码子简并性，与大肠杆菌gal酶相比，这些多肽的推导氨基酸序列高度保守（同源性大于90%）。这些研究确定了两种医学上重要的沙门氏菌物种gal基因的基本遗传参数，我们的发现支持了大肠杆菌和沙门氏菌属从共同祖先亲本细菌假设的趋异进化。
