  We study the effect of intersegment interactions on the effective bending and
stretching moduli of a semiflexible polymer chain with a finite stretching
modulus. For an interaction potential of a screened Debye-H\" uckel type
renormalization of the stretching modulus is derived on the same level of
approximation as the celebrated Odijk-Skolnick-Fixman result for the bending
modulus. The presence of mesoscopic intersegment interaction potentials couples
the bending and stretching moduli in a manner different from that predicted by
the macroscopic elasticity theory. We advocate a fundamental change in the
perspective regarding the dependence of elastic moduli of a flexible
polyelectrolyte on the ionic conditions: stretchability. Not only are the
persistence length as well as the stretching modulus dependent on the salt
conditions in the solution, they are fundamentally coupled via the mesoscopic
intersegment interaction potential. The theory presented here compares
favorably with recent experiments on DNA bending and stretching.
