Protist-Type Lysozymes of the Nematode Caenorhabditis elegans Contribute to Resistance against Pathogenic Bacillus thuringiensis

Pathogens represent a universal threat to other living organisms. Most organisms express antimicrobial proteins and peptides, such as lysozymes, as a protection against these challenges. The nematode Caenorhabditis elegans harbours 15 phylogenetically diverse lysozyme genes, belonging to two distinct types, the protist- or Entamoeba-type (lys genes) and the invertebrate-type (ilys genes) lysozymes. In the present study we characterized the role of several protist-type lysozyme genes in defence against a nematocidal strain of the Gram-positive bacterium Bacillus thuringiensis. Based on microarray and subsequent qRT-PCR gene expression analysis, we identified protist-type lysozyme genes as one of the differentially transcribed gene classes after infection. A functional genetic analysis was performed for three of these genes, each belonging to a distinct evolutionary lineage within the protist-type lysozymes (lys-2, lys-5, and lys-7). Their knock-out led to decreased pathogen resistance in all three cases, while an increase in resistance was observed when two out of three tested genes were overexpressed in transgenic lines (lys-5, lys-7, but not lys-2). We conclude that the lysozyme genes lys-5, lys-7, and possibly lys-2 contribute to resistance against B. thuringiensis, thus highlighting the particular role of lysozymes in the nematode's defence against pathogens.

病原体对所有生命体均构成普遍威胁。多数生物体可表达抗菌蛋白与肽类（如溶菌酶(lysozyme)）以抵御此类侵染。秀丽隐杆线虫（Caenorhabditis elegans）拥有15个系统发育多样性各异的溶菌酶基因，可分为两类截然不同的亚型：原生生物型/内阿米巴型（lys基因）与无脊椎动物型（ilys基因）溶菌酶。本研究系统表征了数种原生生物型溶菌酶基因在抗革兰氏阳性细菌苏云金芽孢杆菌（Bacillus thuringiensis）杀线虫菌株的免疫防御中的作用。通过基因表达微阵列及后续的实时定量反转录聚合酶链反应（qRT-PCR）分析，我们发现感染后原生生物型溶菌酶基因属于差异转录的基因类别之一。我们针对其中3个分别隶属于原生生物型溶菌酶不同进化分支的基因（lys-2、lys-5与lys-7）开展了功能遗传学分析：敲除这三个基因后，线虫的病原体抗性均出现下降；而在转基因品系中过表达3个受试基因中的2个（lys-5与lys-7，而非lys-2）时，线虫的抗性则有所提升。综上，溶菌酶基因lys-5、lys-7以及可能的lys-2均可增强线虫对苏云金芽孢杆菌的抗性，这凸显了溶菌酶在秀丽隐杆线虫抗病原体免疫防御中的独特作用。