大肠杆菌丝氨酸羟甲基转移酶基因(glyA)的克隆和表达

利用插入失活及营养缺陷型互补法将大肠杆菌K12 13kb的glyA基因克隆到质粒pBR329中。将重组质粒酶切,亚克隆,确定2.6kb PstI-EcoRI亚克隆片段带有完整的glyA基因。共获得12株glyA基因重组菌,对重组质粒进行了酶切鉴定。不同重组菌丝氨酸羟甲基转移酶(SHMT)活性及其酶表达量均不相同。受体菌未检测到丝氨酸的产生。重组菌株JM109(pSM13)、K12(pSM13)、K12(pSM14)和K12(pSM15)SHMT酶表达量分别占全菌可溶性蛋白的15.7％、15.4％、11.8％和9.48％。     

直接利用糖质原料产L-丝氨酸谷氨酸棒杆菌glyA基因序列分析

以能够利用糖质原料产L-丝氨酸的谷氨酸棒杆菌（Corynebacterium glutamicum）SYPS-062 glyA基因为研究对象,比较cglutamicum SYPS-062与C．glutamicum模式菌株ATCC13032的glyA基因的异同。分别以SYPS-062及ATCC13032基因组为模板,利用PCR技术获得丝氨酸羟甲基转移酶编码基因glyA。核苷酸序列分析结果表明,来源于SYPS-062和ATCC13032的glyA基因片段全长均为1305bp,编码434个氨基酸,分子量为46．5kD,基因的同源性为99．54％,存在6个核苷酸的差异,引起一个氨基酸残基的突变。将获得的基因分别在大肠杆菌（Escherichia coli）BL21（DE3）中诱导表达。酶活测定结果显示,2种不同菌株来源的重组SHMT的比活力稍有差异,说明SYPS-062 glyA基因的差异对其表达产物SHMT蛋白构型及功能影响不大。提示对于C.glutamicum SYPS-062能够利用糖质原料产L-丝氨酸的机制解析应进一步从glyA基因的转录水平、翻译水平及胞内SHMT辅酶的供给情况等方面进行深入研究探讨。     

甲醇利用菌SDM11中glyA基因的克隆及其在大肠杆菌中的表达

对甲醇降解菌Methylobacterium.sp SDM11中的glyA基因进行克隆及特性研究,以获得更多的丝氨酸羟甲基转移酶(serine hydroxymethyltransferase,SHMT)资源。根据GenBank中已报道的Methylobacterium extorquensAM1中的glyA基因序列(登录号:L33463)设计引物,以SDM11的基因组DNA为模板,PCR扩增glyA基因。利用pETblue-2载体将该基因在大肠杆菌BL21(DE3)中得到表达。PCR扩增到一个1.40 kb大小的DNA片段,经过blast软件比对分析,发现该片段与已报道的Methylobacterium extorquensAM1的glyA基因的序列相似性为95%,氨基酸序列的相似性为98%。该基因编码468个氨基酸,预测的分子量大小为52.2 kD,等电点为7.02,发现纯化后的目标蛋白具有SHMT酶活性,并初步测定了酶活力。     

两种菌株来源的glyA基因的克隆、表达及酶活性检测

采用PCR方法,分别从大肠杆菌和嗜热链球菌基因组DNA中扩增获得glyA基因,分别克隆入载体pET-28 a(+)中并进行表达,分离和纯化得到两种不同来源的SHMT,分别检测两种SHMT的逆向酶活。比较来源于大肠杆菌K12与嗜热链球菌AS1.2471中的glyA基因表达的丝氨酸羟甲基转移酶(SHMT)的活性,以获得高活性的SHMT。结果成功获得两种菌中的glyA基因,并表达出具有较高活性的SHMT,其中嗜热链球菌中glyA基因表达出的SHMT的酶活性大约为大肠杆菌的两倍。从嗜热链球菌中克隆表达的SHMT具有更高的催化活性及良好的工业应用前景。     

植物丝氨酸羟甲基转移酶基因研究进展

丝氨酸羟甲基转移酶(SHMT)是一个含有磷酸吡哆醛(PLP)的四聚体的蛋白质,在亚甲基四氢叶酸(CH2-THF)存在时催化丝氨酸和四氢叶酸生成甘氨酸和N5,N10-甲基四氢叶酸的可逆反应.SHMT在高等植物的一碳代谢和光呼吸中起着非常重要的作用,最近随着植物SHMT纯化技术的进步和重要模式植物基因组测序的完工,使很多植物SHMT基因被克隆出来,并对它们的结构和功能及表达调控进行研究.     

水稻线粒体丝氨酸羟甲基转移酶基因的电子克隆

采用基于EST的电子克隆方法得到了一段长1611bp的cDNA序列,以此为信息探针搜索HTGs数据库,找到一个与之高度匹配的基因组DNA序列——OSJNBa0057G07克隆。用FGENESH分析该克隆中的联配区域得到一个包含14个外显子和13个内含子的基因。该基因位于水稻第3染色体物理图谱的136．0～137．6cM区域。推导的ORF编码498个氨基酸,经BLASTP搜索SWISS-PROT数据库和蛋白序列的亚细胞定位显示,该基因编码水稻的线粒体丝氨酸羟甲基转移酶(mSHMT)。该基因受到EST序列的完全支持,其中不乏来自盐胁迫、稻瘟病菌侵染等逆境处理的EST序列,推测该基因与水稻对逆境的应答反应有关。     

氨基脱氧分支酸合成酶对Corynebacterium glutamicum SYPS-062积累L-丝氨酸的影响

为阐明氨基脱氧分支酸合成酶(ADC合成酶)在Corynebacterium glutamicum SYPS-062体内积累L-丝氨酸过程中的作用,通过交叉PCR以及同源重组的方法敲除叶酸途径关键酶ADC合成酶的编码基因pabAB,构建了叶酸缺陷型菌株Corynebacterium glutamicum SYPS-062△pabAB,同时构建pabAB基因增强表达重组菌C.glutamicum SYPS-062(pJC Ⅰ-pabAB).分别考察了ADC合成酶对菌株生长的影响、对L-丝氨酸降解途径关键酶丝氨酸羟甲基转移酶(SHMT)的影响以及其对L-丝氨酸积累的影响.结果表明,与出发菌株相比,增强表达基因pabAB重组菌的ADC合成酶的酶活力提高了33%.SHMT酶的酶活力提高了30%,其最大比生长速率(μm)提高了48%,单位细胞产酸率(Yp/x)降低了36.2%;而敲除基因pabAB重组菌的ADC合成酶的酶活力降低了61%.SHMT酶的酶活力降低了20%,最大比生长速率降低了32%,单位细胞产酸率提高了12%.     

shPLCε通过YAP抑制前列腺癌细胞的丝氨酸/甘氨酸代谢和增殖 *

目的 该研究旨在探讨磷脂酰肌醇特异性磷脂酶C epsilon(phospholipase C epsilon, PLCε)对前列腺癌细胞丝氨酸/甘氨酸代谢及细胞增殖的影响。方法 慢病毒及质粒转染LNCAP、PC3细胞,q-PCR、Western blot分别检测LNCAP、PC3细胞中 PLCε、Yes相关蛋白(yes associated protein,YAP)、丝氨酸/甘氨酸生成酶[包括磷酸丝氨酸转氨酶1(phosphoserine aminotransferase1,PSAT1)、磷酸丝氨酸磷酸酶(phosphoserine phosphatase,PSPH)、丝氨酸羟甲基转移酶2(serine hydroxymethyltransferase2,SHMT2)及增殖相关基因细胞周期蛋白D1(Cyclin D1)、增殖细胞核抗原(proliferating cell nuclear antigen,PCNA)]的表达情况;克隆形成实验及MTT实验检测细胞的克隆形成率及增殖活性。结果 (1)感染LV-shPLCε可显著下调前列腺癌细胞LNCAP、PC3中的PLCε、YAP、PSAT1、PSPH、SHMT2及增殖相关基因的mRNA及蛋白质水平,同时抑制细胞的克隆形成能力和增殖活性;(2)在shPLCε组细胞中加入过表达YAP质粒后,能明显逆转YAP、PSAT1、PSPH、SHMT2及增殖相关基因的下调,但加入干扰YAP质粒后结果相反。结论 shPLCε可通过下调YAP的表达抑制前列腺癌细胞的丝氨酸/甘氨酸生成,从而抑制细胞的增殖。     

利用丝氨酸羟甲基转移酶和β-酪氨酸酶偶联反应合成L-酪氨酸

本文研究了在5‘-磷酸吡哆醛和四氢叶酸存在条件下,通过丝氨酸羟甲基转移酶和β-酪氨酸酶偶联反应,由甘氨酸、甲醛和酚合成L-酪氨酸的反应条件。重组产气克雷伯氏菌菌株(Klebsiella aerogenes)和草生欧文氏菌(Erwinia her-hicola)(ATCC21434)分别为丝氨酸羟甲基转移酶和β-酪氨酸酶的来源。加入到反应器中的酚和甲醛的量根据反应的需求,应以这些化合物使酶失活降低到最小为准。高浓度的四氢叶酸用来与甲醛形成复合物。整个反应的最适pH为7.0左右,在这个pH范围内甘氨酸对β-酪氨酸酶的抑制作用最小。由于相同原因,有意思地维持甘氨酸的初始浓度在较低水平。反应混合物(500ml)含0.25M甘氨酸,0.5mM 5’-磷酸吡哆醛,0.056Mβ巯-基乙醇,7mM甲氢叶酸及初始酚浓度0.32%,细胞于pH7.0,37℃培养。加入37%的甲醛使反应开始。酚和甲醛的浓度和添加比例应经常检查和调整。反应6小时后的,L-酪氨酸产率77.3%(26.3克/升),甘氨酸转化率6.4%。     

假单胞菌N-13中丝氨酸羟甲基转移酶的酶学性质研究

从产L-丝氨酸菌株假单胞菌N-13中纯化了丝氨酸羟甲基转移酶,并对其性质进行了研究.结果表明,丝氨酸羟甲基转移酶酶活力在pH=7.0～9.0间稳定,最适宜pH=8.0;酶的最适温度为35℃,在30～40℃水浴30 min酶活力未见明显下降.磷酸吡哆醛的最适添加浓度为25 μmol·L-1.研究了不同金属离子对酶活力的影...     

Selection of SALMONELLA TYPHIMURIUM Mutants with Altered Serine Transhydroxymethylase Regulation

In Salmonella typhimurium the glyA gene product, serine transhydroxymethylase (E.C. 2.1.2.1.; L-serine:tetrahydrofolate-5,10-hydroxymethyltransferase) is responsible for the interconversion of serine and glycine. This reaction also provides the cell with one-carbon units from the 5,10-methylene-tetrahydrofolate formed during glycine synthesis. Despite the importance of this enzyme, however, no mutants in which its regulation has been specificially altered have been isolated. To isolate such mutants, we have devised a selection procedure using a strain (glyA951) in which the serine transhydroxymethylase activity is reduced. When this enzyme is completely repressed, the mutant requires gylcine for growth. Revertants which retain the glyA951 lesion, but no longer require glycine, have been isolated and the serine transhydroxymethylase regulation examined. One revertant has a 7-fold elevated serine transhydroxymethylase level, which can be repressed the normal amount (about 5-fold) when the cells are grown in supplemented media. Another revertant has only a 2-fold higher serine transhydroxymethylase level; however, the amount of repression is reduced. The new lesions in both mutants cotransduce with the glyA gene and are distinct from other mutations that alter the regulation of both serine transhydroxymethylase and the methionine biosyntheitc enzymes.     

Regulation of the Escherichia coli glyA gene by the purR gene product.

The purine regulon repressor protein, PurR, was shown to be a purine component involved in glyA regulation in Escherichia coli. Expression of glyA, encoding serine hydroxymethyltransferase activity, was elevated in a purR mutant compared with a wild-type strain. When the purR mutant was transformed with a plasmid carrying the purR gene, the serine hydroxymethyltransferase levels returned to the wild-type level. The PurR protein bound specifically to a DNA fragment carrying the glyA control region, as determined by gel retardation. In a DNase I protection assay, a 24-base-pair region was protected from DNase I digestion by PurR. The glyA operator sequence for PurR binding is similar to that reported for several pur regulon genes.     

Fatty acid synthetase, malic enzyme and other NADP+ binding dehydrogenases have similar antigenic determinant(s) at the NADPH binding domain

The sequence of tryptic and chymotryptic peptides from cytosolic and mitochondrial rabbit liver serine hydroxymethyltransferase are compared to the proposed sequence of a protein coded for by the glyA gene of Escherichia coli. The E. coli glyA gene is believed to code for serine hydroxymethyltransferase. Extensive sequence homology between these peptides were found for the proposed E. coli enzyme in the aminoterminal two-thirds of the molecule. All three proteins have identical sequences from residue 222-231. This sequence is known to contain the lysyl residue which forms a Schiff's base with pyridoxal-P in the two rabbit liver enzymes. These results support the interpretation that the proposed sequence of E. coli serine hydroxymethyltransferase is correct. The data also show that cytosolic and mitochondrial serine hydroxymethyltransferase are homologous proteins.     

Metabolic engineering of acetaldehyde production by Streptococcus thermophilus

The process of acetaldehyde formation by the yogurt bacterium Streptococcus thermophilus is described in this paper. Attention was focused on one specific reaction for acetaldehyde formation catalyzed by serine hydroxymethyltransferase (SHMT), encoded by the glyA gene. In S. thermophilus, SHMT also possesses threonine aldolase (TA) activity, the interconversion of threonine into glycine and acetaldehyde. In this work, several wild-type S. thermophilus strains were screened for acetaldehyde production in the presence and absence of L-threonine. Supplementation of the growth medium with L-threonine led to an increase in acetaldehyde production. Furthermore, acetaldehyde formation during fermentation could be correlated to the TA activity of SHMT. To study the physiological role of SHMT, a glyA mutant was constructed by gene disruption. Inactivation of glyA resulted in a severe reduction in TA activity and complete loss of acetaldehyde formation during fermentation. Subsequently, an S. thermophilus strain was constructed in which the glyA gene was cloned under the control of a strong promoter (P(LacA)). When this strain was used for fermentation, an increase in TA activity and in acetaldehyde and folic acid production was observed. These results show that, in S. thermophilus, SHMT, displaying TA activity, constitutes the main pathway for acetaldehyde formation under our experimental conditions. These findings can be used to control and improve acetaldehyde production in fermented (dairy) products with S. thermophilus as starter culture.     

Genetics of the serine cycle in Methylobacterium extorquens AM1: cloning, sequence, mutation, and physiological effect of glyA, the gene for serine hydroxymethyltransferase.

The gene (glyA) of Methylobacterium extorquens AM1 encoding serine hydroxymethyltransferase (SHMT), one of the key enzymes of the serine cycle for C1 assimilation, was isolated by using a synthetic oligonucleotide with a sequence based on amino acid sequence conserved in SHMTs from different sources. The amino acid sequence deduced from the gene revealed high similarity to those of known SHMTs. The cloned gene was inactivated by insertion of a kanamycin resistance gene, and recombination of this insertion derivative with the wild-type gene produced an SHMT null mutant. Surprisingly, this mutant had lost its ability to grow on C1 as well as on C2 compounds but was still able to grow on succinate. The DNA fragment containing glyA was shown not to be linked with fragments carrying serine cycle genes identified earlier, making it the fourth chromosomal region of M. extorquens AM1 to be indicated as being involved in C1 assimilation.     

Identification of glyA as a symbiotically essential gene in Bradyrhizobium japonicum

A Bradyrhizobium japonicum Tn5 mutant (strain 3160) induced numerous, tiny, white nodules which were dispersed over the whole root system of its natural host plant, soybean (Glycine max). These ineffective, nitrogen non-fixing pseudonodules were disturbed at a very early step of bacteroid and nodule development. Subsequent cloning and sequencing of the DNA region mutated in strain 3160 revealed that the Tn5 insertion mapped in a gene that had 60% homology to the Escherichia coli glyA gene coding for serine hydroxymethyltransferase (SHMT; E.C.2.1.2.1.). SHMT catalyses the biosynthesis of glycine from serine and the transfer of a one-carbon unit to tetrahydrofolate. The B. japonicum glyA region was able to fully complement the glycine auxotrophy of an E. coli glyA deletion strain. Although the Tn5 insertion in B. japonicum mutant 3160 disrupted the glyA coding sequence, this strain was only a bradytroph (i.e. a leaky auxotroph). Thus, B. japonicum may have an additional pathway for glycine biosynthesis. Nevertheless, the glyA mutation was responsible for the drastic symbiotic phenotype visible on plants. It may be possible, therefore, that a sufficient supply with glycine and/or a functioning C1-metabolism are indispensable for the establishment of a fully effective, nitrogen-fixing root nodule symbiosis.