Self-protection against cell wall hydrolysis in Streptococcus milleri NMSCC 061 and analysis of the millericin B operon
Self-protection against cell wall hydrolysis in Streptococcus milleri NMSCC 061 and analysis of the millericin B operon
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DOI：
10.1128/aem.67.9.3888-3896.2001
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发表时间：
2001-09-01
影响因子：
4.4
通讯作者：
Hastings, JW
中科院分区：
文献类型：
--
作者：
Beukes, M;Hastings, JW
米勒链球菌NMSCC 061产生一种内肽酶——米勒蛋白B，它能水解敏感细胞壁肽聚糖的肽段。4.9 kb染色体区域的核苷酸序列显示3个开放阅读框（orf）和一个推测的tRNA（Leu）序列。这三个orf编码一种微球蛋白B前蛋白（MiIB）、一种推定的免疫蛋白（MiIf）和一种推定的转运蛋白（MiIT）。milB基因编码一个277个氨基酸的前蛋白和一个18个氨基酸的信号肽，具有一致的IIGG切割基序。milT编码的预测蛋白与几种细菌素系统的ABC （atp结合盒）转运蛋白同源，也与大肠杆菌溶血素a的信号序列无关输出蛋白同源。这些相似性强烈表明，milT基因产物参与了米勒霉素b的易位。它们参与了甘氨酸向五肽肽聚糖前体的加成。milleri NMSCC 061细胞壁黏肽（对millericin B裂解具有抗性）与milleri NMSCC 051细胞壁黏肽（敏感）的比较显示出单氨基酸差异。milleri NMSCC 051在含有增加亮氨酸浓度的细胞壁最小培养基中连续生长，导致细胞壁黏肽中的亮氨酸取代了苏氨酸。milleri菌株NMSCC 051细胞壁变体（敏感）在其肽聚糖交叉桥中含有一个氨基酸取代（亮氨酸取代苏氨酸），对millericin b表现出部分敏感性。推测位于milB上游的tRNA（Leu）序列可能是细胞壁特异性tRNA，可能与milF蛋白一起，在亮氨酸添加到五肽肽聚糖前体中发挥潜在作用。有助于对生产者菌株中的千粉蛋白B进行自我保护。
Streptococcus milleri NMSCC 061 produces an endopeptidase, millericin B, which hydrolyzes the peptide moiety of susceptible cell wall peptidoglycan. The nucleotide sequence of a 4.9-kb chromosomal region showed three open reading frames (ORFs) and a putative tRNA(Leu) sequence. The three ORFs encode a millericin B preprotein (MiIB), a putative immunity protein (MiIf), and a putative transporter protein (MiIT). The milB gene encodes a 277-amino-acid preprotein with an 18-amino-acid signal peptide with a consensus IIGG cleavage motif. The predicted protein encoded by milT is homologous to ABC (ATP-binding cassette) transporters of several bacteriocin systems and to proteins implicated in the signal-sequence-independent export of Escherichia coli hemolysin A. These similarities strongly suggest that the milT gene product is involved in the translocation of millericin B. The gene milF encodes a protein of 302 amino acids that shows similarities to the FemA and FemB proteins of Staphylococcus aureus, which are involved in the addition of glycine to a pentapeptide peptidoglycan precursor. Comparisons of the cell wall mucopeptide of S. milleri NMSCC 061(resistant to lysis by millericin B) and S. milleri NMSCC 051 (sensitive) showed a single amino acid difference. Serial growth of S. milleri NMSCC 051 in a cell wall minimal medium containing an increased concentration of leucine resulted in the in vivo substitution of leucine for threonine in the mucopeptide of the cell wall. A cell wall variant of S. milleri NMSCC 051 (sensitive) that contained an amino acid substitution (leucine for threonine) within its peptidoglycan cross bridge showed partial susceptibility to millericin B. The putative tRNA(Leu) sequence located upstream of milB may be a cell wall-specific tRNA and could together with the milF protein, play a potential role in the addition of leucine to the pentapeptide peptidoglycan precursor and thereby, contributing to self-protection to millericin B in the producer strain.