What is indicated by ‘q’ in the figure?

In lac operon structural genes lac Z ,lac Y and lac A undergoes transcription and forms single mRNA. This mRNA undergoes translation and forms enzymatic proteins beta-galactosidase , permease and transacetylase. Here single mRNA is responsible for synthesis of three proteins ,so it is polycistronic . lac z part of the gene synthesizing beta - galactosidase ,lac Y part of the gene synthesizing--permease and lac A part of the gene synthesizing ---transacetylase