Required for 40S ribosome biogenesis. Involved in nucleolar processing of pre-18S ribosomal RNA and ribosome assembly. Binds to RNA. Required for female germline development, cell viability during eye development and for survival of dividing cells and epithelial cells during early wing disk development. Monomer. Component of the ribosomal small subunit (SSU) processome. Exogenous expression in embryos shows a predominant localization in nucleoplasm in some epidermal cells while most other cells show perinucleolar ring structure. In heat-shocked third instar larvae, localization varies among cells. In gut cells, some show localization in both nucleoplasm and nucleolus while in others localization is restricted to the center of the fibrillarin-positive part of the nucleolus, in the fibrillar center, where rRNA transcription occurs. On ectopic expression, a nuclear engrailed pattern is observed in embryonic epidermal cells. Expression is detected in nurse cells during oogenesis throughout stage 10A and persists through stage 14 with some expression in the anterior part of the oocyte. Expression is ubiquitously detected at all stages of embryogenesis. Observed in salivary gland cells from heat shocked transgenic third instar larvae. Homozygous mutant larvae show arrested development at the first instar stage as they fail to increase in size or develop into the second instar larval stage and die 2-3 days after hatching, without morphological defect. Mutant larvae show an overall reduction of most intermediate and mature rRNA as a consequence of abnormal pre-rRNA processing. Mutants show defects in rRNA processing and aberrant pre-rRNA species. An abnormal cleavage in the 3'-end of the pre-rRNA occurs within the presumptive 28S rRNA but it does not lead to accumulation of a truncated 28S rRNA. Belongs to the KRR1 family.