Six independent transformation experiments of tobacco leaf explants were carried out using two different constructs - (1) construct I containing only a Hygromycin-resistance gene $(HYG^R)$ as a selection marker and (2) construct II with the $(HYG^R)$ gene as a selection marker and a passenger gene encoding a dephosphorylase enzyme $(DEP)$ that inhibits phosphorylation in cells in which it is expressed. The marker as well as passenger genes were placed under transcriptional control of a constitutive promoter $(CaMV35S)$, which expresses in all cells/tissues. The results obtained are tabulated below.
Explain how The $HYG^R$ gene used in construct II lost its functionality probably due to mutation(s) either in the gene or in the promoter.
My approach: There is a probability that Expression of the $DEP$ gene is lethal to the transformed cells.