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. To survive, grow and reproduce organisms need proteins that function as enzymes, messengers, structural components of intra-and intercellular structures, transport and storage molecules. In addition to proper functionality, proteins have to be available in cells at the right time, place and in adequate amount to ensure/contribute to organism's fitness/viability. Therefore, regulation of protein synthesis-translation-is of central importance in molecular biology. Translation is a highly complex process comprising an mRNA, aminoacylated tRNA-s, several protein factors and ribosomes and it can be regulated by various mechanisms 1,2. An emerging paradigm is ribosome composition mediated translational control 3 that is based on the hypothesis that ribosome can regulate translation via its composition. This concept is based on the accumulating evidence that eukaryotic and bacterial cells produce heterogeneous ribosomes, i.e. ribosomes containing alternative components 4,5. Ribosome heterogeneity has various sources: rRNA sequence and modifications, ribosomal protein (r-protein) stoichiometry, modifications and paralogs 6. Therefore, the potential of creating ribosomes with various composition is enormous 7. An intriguing question is whether ribosome heterogeneity is functional. It means that first, biochemically different ribosomes are present in vivo under adverse growth conditions and second, that these variant ribosomes affect cell physiology via protein synthesis 8. Recently, functional ribosome heterogeneity has been defined as "variations in ribosome composition that influence its activity, thereby changing the output of translation" 9. As translation initiation is the main regulatory checkpoint for protein synthesis most research on the functionality of ribosome heterogeneity has been focused on this step of translation 6. Specifically, one possible viewpoint to ribosome composition mediated translational control has been ribosome filter hypothesis stating that ribosomes may d...