that's difficlult to say. Best, you screen the literature (e.g. in [Only registered users see links. ]) for 'directed evolution'. Look out for the reviews /
papers / articles by Willem P. Stemmer, Frances H. Arnold or Andreas
Pluckthun to get some ideas on current methods of mutagenesis and
screening. I am just afraid that there are as many methods as there are
papers (almost, of course). You will have to adapt them on your problem.
Alternatively, you simply could try a deliberated minimally dosed UV
irradiation of freshly seeded bacteria on agar plates and then screen the
colonies if you have an easy to perform assay (non-destructive on the plate
or in colony-lift style) for keratinase.
How do you plan to screen for positives and what kind of improvement do you