College of Marine Science, University of South Florida, St. Petersburg, Florida, United States of America.
PLoS One. 2012;7(8):e43506. doi: 10.1371/journal.pone.0043506. Epub 2012 Aug 15.
Microbial genomic sequence analyses have indicated widespread horizontal gene transfer (HGT). However, an adequate mechanism accounting for the ubiquity of HGT has been lacking. Recently, high frequencies of interspecific gene transfer have been documented, catalyzed by Gene Transfer Agents (GTAs) of marine α-Proteobacteria. It has been proposed that the presence of bacterial genes in highly purified viral metagenomes may be due to GTAs. However, factors influencing GTA-mediated gene transfer in the environment have not yet been determined. Several genomically sequenced strains containing complete GTA sequences similar to Rhodobacter capsulatus (RcGTA, type strain) were screened to ascertain if they produced putative GTAs, and at what abundance. Five of nine marine strains screened to date spontaneously produced virus-like particles (VLP's) in stationary phase. Three of these strains have demonstrated gene transfer activity, two of which were documented by this lab. These two strains Roseovarius nubinhibens ISM and Nitratireductor 44B9s, were utilized to produce GTAs designated RnGTA and NrGTA and gene transfer activity was verified in culture. Cell-free preparations of purified RnGTA and NrGTA particles from marked donor strains were incubated with natural microbial assemblages to determine the level of GTA-mediated gene transfer. In conjunction, several ambient environmental parameters were measured including lysogeny indicated by prophage induction. GTA production in culture systems indicated that approximately half of the strains produced GTA-like particles and maximal GTA counts ranged from 10-30% of host abundance. Modeling of GTA-mediated gene transfer frequencies in natural samples, along with other measured environmental variables, indicated a strong relationship between GTA mediated gene transfer and the combined factors of salinity, multiplicity of infection (MOI) and ambient bacterial abundance. These results indicate that GTA-mediated HGT in the marine environment with the strains examined is favored during times of elevated bacterial and GTA abundance as well as in areas of higher salinity.
微生物基因组序列分析表明存在广泛的水平基因转移（HGT）。然而，一直缺乏一种充分的机制来解释 HGT 的普遍性。最近，人们记录到海洋α-变形菌的基因转移剂（GTAs）催化的种间基因转移的高频发生。有人提出，高度纯化的病毒宏基因组中细菌基因的存在可能是由于 GTAs 所致。然而，影响环境中 GTA 介导基因转移的因素尚未确定。一些含有与红细菌（RcGTA，模式株）相似的完整 GTA 序列的全基因组测序株被筛选出来，以确定它们是否产生了推定的 GTAs，以及产生了多少 GTA。迄今为止，从 9 株海洋菌株中筛选出的 5 株在静止期自发产生类似病毒的粒子（VLP）。其中 3 株表现出基因转移活性，其中 2 株由本实验室记录。这两个菌株 Roseovarius nubinhibens ISM 和 Nitratireductor 44B9s 被用来产生 GTA，分别命名为 RnGTA 和 NrGTA，并在培养物中验证了基因转移活性。从标记供体菌株中纯化的 RnGTA 和 NrGTA 颗粒的无细胞制剂与自然微生物群落孵育，以确定 GTA 介导的基因转移水平。同时，还测量了几个环境参数，包括噬菌体诱导指示的溶原性。在培养系统中，GTA 的产生表明大约一半的菌株产生了 GTA 样颗粒，最大的 GTA 计数范围为宿主丰度的 10-30%。对自然样品中 GTA 介导的基因转移频率进行建模，以及其他测量的环境变量，表明 GTA 介导的基因转移与盐度、感染倍数（MOI）和环境细菌丰度等综合因素之间存在很强的关系。这些结果表明，在所研究的菌株中，GTA 介导的 HGT 在细菌和 GTA 丰度升高以及盐度较高的区域更为有利。
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