Growth, Mutation and Recombination in Bacteriophage

在涉及原核生物或真核生物的杂交中，遗传交换可能是非随机分布的，即它们可能表现出“干扰”。对两个明显存在干扰的系统的研究将继续进行。在这两种系统中，干扰可能依赖于沿染色体移动的“机器”。对这些机器的了解将有助于我们对基因重组的分子机制的理解。(1)大肠杆菌的RecBCD重组途径作用于携带重组热点Chi的噬菌体lambda时，在噬菌体颗粒中回收的重组染色体有时会发生一对交换，一个互反，一个非互反。非互反交换不是随机发生的，而是在标准lambda链接映射上的互反交换的右侧发生的。这种非随机关系的基础在于RecBCD蛋白，它以双链断裂的方式进入DNA，并沿着双链破坏性地移动，当它遇到序列5'GCTGGTGG3' （Chi）时，从核酸外切酶转变为“重组酶”。这两种交流机制的模型以及它们之间的关系正在接受检验。(2)在许多真核生物中，在减数分裂二价中存在一次交换降低了在附近间隔中发现交换的可能性（交叉干扰）。建立了一个简单的模型，将基因转换的观察结果与交叉干扰的观察结果结合起来。该模型假设转换是随机发生的，但是那些有关联交叉的转换总是被几个没有关联交叉的转换分开。因此，这两种重组事件（其中一种有时被称为“互反”而另一种不是）相对于自身和彼此都是非随机分布的。模型的开发和测试仍在继续。关于模型背后机制的假设正在被检验。这种机制可能涉及到一个“机器”，它沿着突触复合体移动，解决双链断裂修复中间物，使得固定数量的修复事件在相邻的交叉之间落下，而不伴有交叉。重组发生在几乎所有的生物体中，它负责导致后代基因的新组合，并保护DNA和基因免受损害。对这一过程的更好理解应该适用于许多生物体，并可能导致开发将外源基因引入生物体的更好方法（“基因工程”）。＊＊＊

Abstract 9402695 In crosses involving either pro- or eukaryotes, genetic exchanges may be nonrandomly distributed with respect to each other, i.e., they may show "interference." The investigation of two systems in which interference is manifest will continue. In both systems, interference may depend on a "machine" that moves along chromosomes. Understanding of these machines will contribute to our understanding of molecular mechanisms of genetic recombination. (1) When the RecBCD recombination pathway of E. coli operates on phage lambda carrying the recombination hot spot Chi, recombinant chromosomes recovered in phage particles have sometimes indulged in a pair of exchanges, one reciprocal and one not. The nonreciprocal exchange occurs not at random but to the right of the reciprocal one on the standard lambda linkage map. The basis for this nonrandom relationship resides in the RecBCD protein, which enters DNA at a double-chain break and travels destructively along the duplex, changing from an exonuclease to a "recombinase" when it encounters the sequence 5'GCTGGTGG3' (Chi). Models for the mechanisms of the two exchanges and for the relationship between them are being tested. (2) In many eukaryotes, the presence of one exchange in a meiotic bivalent reduces the probability of finding an exchange in nearby intervals (chiasma interference). A simple model that unites observations on gene conversion with those on chiasma interference has been developed. The model supposes that conversions occur at random with respect to each other, but that those with associated crossing over are invariably separated by several conversions that lack associated crossing over. Thus, the two kinds of recombination events(one of which is sometimes called "reciprocal" and the other not) are nonrandomly distributed with respect to themselves and each other. Development of the model and testing of it continues. Hypotheses regarding mechanisms underlying the model are being te sted. Such mechanisms may involve a "machine" that travels along the synaptonemal complex resolving double-chain break-repair intermediates such that a fixed number of repair events unaccompanied by crossing over fall between neighboring crossovers. %%% Recombination occurs in almost all organisms, and is responsible for leading to new combinations of genes in offspring as well as protection against damage to DNA and genes. A better understanding of this process should be applicable to many organisms, and could lead to the development of better ways of introducing foreign genes into organisms ("genetic engineering"). ***