A specific protein kinase (PK) gene is speculated to be differentially spliced in muscle tissue. This gene comprises three exons and two intron sequences and in fibroblast cells encodes a 38.5-kD protein. Investigators have transfected various portions of a genomic or cDNA copy of the PK gene (see the constructs in part (a) of the figure) into both muscle and fibroblast cells. The expression system utilizes a promoter (P in Figure a) active in both cell types, and a C-terminal fusion to a small epitope tag called V5, which contributes ~5.5 kD to the mass of the fusion protein. Part (b) of the figure shows the results of a Western blot using anti-V5 antibody, designed to analyze the expression products of the transfected cells. A negative control (Neg) of untransfected muscle cells is included. Molecular weight markers in kD are indicated on the left. (a) 1 2 3 1 1 2 = = 300 bp 3 2 VS VS 3 VS (b) 45->> 29>> 25-> Fibroblast Muscle Neg 1 2 3 1 2 3 What can be concluded about differential splicing of the PK gene in fibroblast versus muscle cells? O in muscle cells, the translated mRNA from construct 1 contains only exon 3. There is no evidence of differential splicing of construct 1 in fibroblasts and muscle cells. O In fibroblasts, the translated mRNA from construct 1 contains exons 1, 2, and 3. O in muscle cells, only exons 1 and 2 from construct 1 are included in the translated mRNA.