A culture of E. coli (leu+, thr+, azi-r) is grown and infected by the lysogenic
ID: 55924 • Letter: A
Question
A culture of E. coli (leu+, thr+, azi-r) is grown and infected by the lysogenic phage P1. The resulting lysate is used to infect a similar culture of E. coli (leu-, thr-, azi-s). Cells that survive the infection are plated on minimal medium and on medium supplemented with either leucine or threonine. In this way, bacteria carrying leu+, thr+, or both can be selected and tested for the presence of other markers.
Selected marker
leu+
thr+
leu+ & thr+
Frequency of unselected marker
50% azi-r & 2% thr+
3% leu+ & thr+ 0% azi-r
0% azi-r
a) Which two markers are closely linked?
b) What is the linear order of the three markers?
c) The data suggest that the region thr-leu represents the extreme length of donor fragment which can be transduced by P1. Why?
Explanation / Answer
A). off the leu+ cells 50% are azi R while 2 % are thr + from this we can conclude that azi R and leu + are near to each other and that of leu+ and thr+are apart.
select the thr+ cells and then determine how many of them have two markers, 30% leu+ and 0%azi R, by this we can conclude that thr+ is closer to leu than it is to aziR because thr+ and aziR are so far apart that they are never con-transducted.
B). The linear order of the marker should be thr---leu----azi.