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We mapped polyoma virus-specific mRNAs isolated from productively infected mouse 3T6 cells on the viral genome by analyzing nuclease St-resistant RNA-DNA hybrids. The polyoma early mRNAs, which code for the three T antigens, have several 5' ends near 73 map units (m.u.). During the late phase of infection an additional 5' end is found near 71 m.u. All of the major early mRNAs have common 3' ends at 26.01 m.u. There is a minor species of early mRNA with a 3' end at 99.05 m.u. There are two proximal and two distal splice junctions in the early region which are used to generate three different spliced early mRNAs. There are three late mRNAs encoding the three virion proteins, VP1, VP2, and VP3. The late mRNAs have common 3' ends at 25.34 m.u. The late mRNAs have heterogeneous 5' leader sequences derived from the region between 65.53 and 68.42 m.u. The leader sequences are joined to the bodies of the messages coding for VP2, VP3, and VP1 at 66.59, 59.62, and 48.57 m.u., respectively. These results confirm and extend previous analyses of the fine structure of polyoma mRNAs.

Raising the concentration of phenylalanine and other amino acids in MEM leads to the inhibition of growth and in some cases to death of A9. Balb 3T3 , SV40 Balb 3T3 (SVT2), CHO, and WI38. All cells tested exhibited some similar senstivities to certain of the amino acids. but there were some unique differences. Phenylalanine-resistant mutants (Pher) of A9 were isolated that had modified phenylalanine-transport properties. These mutants can be isolated by a single-step selection procedure. A Lineweaver-Burk plot of initial rates of phenylalanine uptake by A9 and mutants showed a biphasic curve suggesting two transport systems. The Pher mutants had altered properties of both systems. It is suggested that the selection of clones resistant to high concentration of several of the natural amino acid may be used as a general method for the isolation of mutants affecting the various amino acid transport systems in mammalian cells.