Ribosomes are essential macromolecular complexes conducting protein biosynthesis in all domains of life. Cells can have heterogeneous ribosomes, i.e. ribosomes with various ribosomal RNA and ribosomal protein (r-protein) composition. However, the functional importance of heterogeneous ribosomes has remained elusive. One of the possible sources for ribosome heterogeneity is provided by paralogous r-proteins. In E. coli, ribosomal protein bL31 has two paralogs: bL31A encoded by rpmE and bL31B encoded by ykgM. This study investigates phenotypic effects of these ribosomal protein paralogs using bacterial strains expressing only bL31A or bL31B. We show that bL31A confers higher fitness to E. coli under lower temperatures. In addition, bL31A and bL31B have different effects on translation reading frame maintenance and apparent translation processivity in vivo as demonstrated by dual luciferase assay. In general, this study demonstrates that ribosomal protein paralog composition (bL31A versus bL31B) can affect cell growth and translation outcome.