The 5’ untranslated region of theEFG1transcript promotes its translation to regulate hyphal morphogenesis inCandida albicans

Abstract

21 22 23 Extensive 5´ untranslated regions (UTR) are a hallmark of transcripts determining hyphal 24 morphogenesis in Candida albicans. The major transcripts of the EFG1 gene, which are 25 responsible for cellular morphogenesis and metabolism, contain a 5´ UTR of up to 1170 nt. 26 Deletion analyses of the 5´ UTR revealed a 218 nt sequence that is required for production 27 of the Efg1 protein and its functions in filamentation, without lowering the level and 28 integrity of the EFG1 transcript. Polysomal analyses revealed that the 218 nt 5´ UTR 29 sequence is required for efficient translation of the Efg1 protein. Replacement of the EFG1 30 ORF by the heterologous reporter gene CaCBGluc confirmed the positive regulatory 31 importance of the identified 5´ UTR sequence. In contrast to other reported transcripts 32 containing extensive 5´ UTR sequences, these results indicate the positive translational 33 function of the 5´ UTR sequence in the EFG1 transcript, which is observed in context of the 34 native EFG1 promoter. The results suggest that the 5´ UTR recruits regulatory factors, 35 possibly during emergence of the native transcript, which aid in translation of the EFG1 36 transcript. 37 38 39 3 IMPORTANCE 40 Many of the virulence traits that make Candida albicans an important human fungal 41 pathogen are regulated on a transcriptional level. Here we report an important regulatory 42 contribution of translation, which is exerted by the extensive 5´ untranslated regulatory 43 sequence (5´ UTR) of the transcript for the protein Efg1, which determines growth, 44 metabolism and filamentation in the fungus. Presence of the 5´ UTR is required for efficient 45 translation of Efg1, to promote filamentation. Because transcripts for many relevant 46 regulators contain extensive 5´ UTR sequences, it appears that virulence of C. albicans 47 48 49 KEYWORDS: Candida albicans; EFG1; filamentation; hyphal morphogenesis; 5´ UTR; 50 posttranscriptional regulation; translation 51 52 Transcriptional networks are known to govern growth and virulence of the human 53 fungal pathogen C. albicans. Transcription factors have been identified that regulate the 54 interconversion between its yeast cell form and a filamentous hyphal form, or the rod-like 55 opaque form. Efg1 is a key bHLH-type regulatory protein that controls hyphal 56 morphogenesis in a dual manner, promoting filamentation under normoxia in the presence 57 of environmental cues (1, 2), but repressing it under hypoxia (3, 4). Its promoting function 58 depends on increased histone acetylation and chromatin remodelling at promoters of target 59 genes (5), which facilitates hyphal initiation; shortly thereafter, however, EFG1 expression is 60 strongly downregulated to prevent its interference with subsequent processes required for 61 hyphal formation (6, 7). Under hypoxia, Efg1 represses the expression of genes encoding 62 hyphal inducers Ace2 and Brg1, thereby downregulating filamentation (8), and it regulates 63 the hypoxia-specific expression of numerous genes. Furthe...