Before revision:  Our prediction are as follows (i) At small concentration of salt the blocked ion current decreases with x. (ii) The effective charge q of DNA piece  grows with x and at q_{b x= 1. (iii) The rate of DNA capture by the channel exponentially grows with x. Our theory is also applicable to translocation of a double stranded DNA in narrow solid state nanopores with positively charged walls.
After revision:  Our prediction are as follows (i) At small concentration of salt the blocked ion current decreases with x. (ii) The effective charge q of DNA piece grows with x and at x= 1. (iii) The rate of DNA capture by the channel exponentially grows with x. Our theory is also applicable to translocation of a double stranded DNA in narrow solid state nanopores with positively charged walls.