Temperature plays an important role in bacteria-host interactions and can be a determining factor for host switching. In this study we sought to investigate the reasons behind growth temperature restriction in the entomopathogenic enterobacterium Photorhabdus. Photorhabdus has a complex dual symbiotic and pathogenic life cycle. The genus consists of 19 species but only one subgroup, previously all classed together as P. asymbiotica, have been shown to cause human disease. These clinical isolates necessarily need to be able to grow at 37 °C, whilst the remaining species are largely restricted to growth temperatures below 34 °C and are therefore unable to infect mammalian hosts. Here, we have isolated spontaneous mutant lines of P. laumondii DJC that were able to grow up to 36 °C - 37 °C. Following whole genome sequencing of 29 of these mutants we identified a single gene, encoding a protein with a RecG-like helicase domain, that for the majority of isolates contained single nucleotide polymorphisms. Importantly, provision of the wild-type allele of this gene in trans restored the temperature restriction, confirming the mutations are recessive, and the dominant effect of the protein product of this gene. The gene appears to be part of a short three cistron operon, which we have termed the Temperature Restricting Locus (TRL). Transcription reporter strains revealed that this operon is induced upon the switch from 30 °C to 37 °C, leading to replication arrest of the bacteria. TRL is absent from all of the human pathogenic species so far examined, although its presence is not uniform in different strains of the P. luminescens subgroup. In a wider context, the presence of this gene is not limited to Photorhabdus, being found in phylogenetically diverse proteobacteria. We therefore suggest that this system may play a more fundamental role in temperature restriction in diverse species, relating to as yet cryptic aspects of their ecological niches and life cycle requirements.

中文翻译：

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昆虫病原菌的温度限制

温度在细菌与宿主之间的相互作用中起着重要作用，并且可以成为宿主转换的决定性因素。在这项研究中，我们试图调查在致病性肠杆菌光杆菌中生长温度受到限制的原因。Photorhabdus具有复杂的共生和致病性双重生命周期。该属由19种组成，但只有一个亚类，以前被全部归类为非对称疟原虫，已被证明可引起人类疾病。这些临床分离株必须能够在37°C下生长，而其余物种在很大程度上限于34°C以下的生长温度，因此无法感染哺乳动物宿主。在这里，我们分离出了能够自发生长到36°C-37°C的laumondii DJC自发突变株。在对这些突变体中的29个进行全基因组测序后，我们确定了一个单一基因，该基因编码具有RecG样解旋酶结构域的蛋白质，大多数分离物均含有单核苷酸多态性。重要的是，反式提供该基因的野生型等位基因恢复了温度限制，证实了突变是隐性的，并且是该基因的蛋白质产物的主要作用。该基因似乎是短的三个顺反子操纵子的一部分，我们将其称为温度限制基因座（TRL）。转录报告基因菌株显示，该操纵子是在从30°C切换到37°C时诱导的，导致细菌的复制停滞。迄今为止，所有人类病原体都没有TRL，尽管在不同的P菌株中TRL的存在并不均匀。发光亚组。在更广泛的背景下，该基因的存在不限于在系统发育多样的蛋白细菌中发现的Photorhabdus。因此，我们建议该系统可能在各种物种的温度限制中发挥更基本的作用，这与其生态位和生命周期要求的隐秘方面有关。