Coren J S, Pierce J C, Sternberg N
DuPont Merck Pharmaceuticals, Glenolden, PA 19036, USA.
J Mol Biol. 1995 May 26;249(1):176-84. doi: 10.1006/jmbi.1995.0287.
Like a variety of other bacteriophages, such as T4 and P22, bacteriophage P1 packages DNA by a "headful" mechanism in which the capacity of the viral capsid determines the size of the single DNA molecule that is packaged. Because of the long-standing and general acceptance of this packaging mechanism, we were surprised to discover that some of our observations, using the in vitro P1 packaging system, could be explained by the packaging of less than headful-sized (< 110 kb) DNA molecules into a P1 capsid. To account for these observations, we describe results that support a model of in vitro P1 packaging in which multiple less than headful-sized DNA molecules are taken into a P1 head until that head has been filled. The results further suggest that the phage so generated can occasionally inject more than one DNA molecule into a cell upon viral infection. The data that supports these conclusions are: (1) the DNAs of the circular P1 cloning vectors pAd10sacBII (32 kb) and pNS358 (14 kb) are packaged in vitro with an efficiency of about 6 to 12% of that of longer concatemers of these DNAs. (2) The in vitro packaging of two differentially marked, less than 18 kb plasmid DNAs in the same reaction results in the production of a phage that can occasionally inject both DNAs into the same cell upon infection. (3) Virus particles generated by the packaging of either pAd10sacBII plasmid DNA or the two differently marked plasmids have a density in CsCl equilibrium gradients that is the same as P1 plaque-forming phage, suggesting that the former phage contain a headful of DNA. These results cannot be explained by Cre-mediated site-specific recombination between plasmids in the P1 packaging extracts. Finally, we present in vivo experiments that are also consistent with the headful packaging of multiple DNAs into a P1 head.
与多种其他噬菌体（如T4和P22）一样，噬菌体P1通过“满头部”机制包装DNA，其中病毒衣壳的容量决定了被包装的单个DNA分子的大小。由于这种包装机制长期以来被广泛接受，我们很惊讶地发现，使用体外P1包装系统进行的一些观察结果，可以通过将小于满头部大小（<110 kb）的DNA分子包装到P1衣壳中来解释。为了解释这些观察结果，我们描述了支持体外P1包装模型的结果，即在体外P1包装模型中，多个小于满头部大小的DNA分子被摄入P1头部，直到该头部被填满。结果还表明，如此产生的噬菌体在病毒感染时偶尔可以将多个DNA分子注入一个细胞。支持这些结论的数据如下：（1）环状P1克隆载体pAd10sacBII（32 kb）和pNS358（14 kb）的DNA在体外被包装，其效率约为这些DNA较长串联体包装效率的6%至12%。（2）在同一反应中对两个差异标记的、小于18 kb的质粒DNA进行体外包装，会产生一种噬菌体，该噬菌体在感染时偶尔可以将两种DNA都注入同一个细胞。（3）由pAd10sacBII质粒DNA或两种差异标记的质粒包装产生的病毒颗粒在CsCl平衡梯度中的密度与P1噬菌斑形成噬菌体相同，这表明前者噬菌体含有一头部的DNA。这些结果不能用P1包装提取物中质粒之间的Cre介导的位点特异性重组来解释。最后，我们展示了体内实验，这些实验也与多个DNA被满头部包装到P1头部的情况一致。
