Hi Nikolay,
Thanks for providing further info. I tried to build a fiber-based structure with 48M bps on my new iMac with 32GM RAM. The fiber program did not exist with the memory allocation error as you saw, but it seems never to finish!
So fiber appears not up to the job, even though in principle it should. Note also that with 48M bps, the PDB format is certainly out of the question. If you really want to go for it, you may try smaller fragments (with the -xml option for the PDBML format) that can be built with fiber, and then assemble them into the whole 48M bps chromosome.
Xiang-Jun